.  .    LIBRARY    .  . 

Connecticut 
Agricultural  College. 

voL...-..._i)-.i.5y — 

CLASS    NO J.....-'....V. 

COST "J.-vvt... 

DATE  iV^^r.^....?. — ISO-*- 


Digitized  by  the  Internet  Archive 

in  2009  with  funding  from 

Boston  Library  Consortium  IVIember  Libraries 


http://www.archive.org/details/vermilionironbeaOOclem 


DEPARTMENT   OF  THE    INTERIOR 


MONOGRAPHS 


OF  THE 


United  States  Geological  Survey 


VOLUME    XLV 


WASHINGTON 

GOVERNMENT     PRINTING     OFFICE 
19  0  3 


'  i ' 


H'J 


UNITED  STATES  GEOLOGICAL  SURVEY 

CHAKLES  P.  WALCOTT,   ])irector 


THE 


VERMILION  IRON-BEARING  DISTRICT  OF  MINNESOTA 


\VITH    AN    ATLAS 


J.  M:oRG^]sr  cle]mej^ts 


CHAELES   RICHARD  VAN  HISE,  Gkologist  in  Charge 


AYASHINGTON 

GOVERNMENT     PRINTING     OFFICE 

1  9  0  3 


^15^ 


CONTENTS. 


Page. 

Letter  of  transmittal 1 ' 

Outline  of  monograph 19 

Chapter  I. — General  description  of  the  district 29 

Introduction 29 

Previous  geologic  work  in  the  district 29 

Scope  of  the  paper 31 

Geographic  limits 32 

Stratigraphy 33 

Physiography  - 34 

Belief 34 

Mesabi  or  Giants  range. 35 

Area  north  and  northwest  of  Giants  range 36 

The  gabbro  plateau 37 

Gunfiint  Lake  area 38 

Drainage 39 

Hydrographic  basin 39 

Streams 40 

Lakes 41 

Differences  in  water  level - 42 

Water  power 43 

.  Origin  of  tlie  lakes 43 

Plankton 46 

Exposures 46 

Forests 47 

Soil 50 

Game  and  fish 50 

Culture 52 

Indian  reservation 54 

Chapter  II. — Resume  of  literature 55 

History  of  exploration  and  character  of  the  routes 56 

Geological  literature , 64 

Chapter  III. — The  Archean 129 

Section  I.  Definition  and  subdivisions 129 

Section  II.  Ely  greenstone 130 

Features  of  the  greenstone 130 

Occurrence  and  character 131 

Distribution - 131 

Exposures 134 

Topography 134 

Structure 135 

Petrographic  characters 136 

The  amygdaloidal  structure 139 

The  spherulitic  structure 141 

The  ellipsoidal  structure 144 

5 


6  CONTENTS. 

CiiAiTER  III. — The  Akchean" — Continued.  Page. 
Section  II.  Ely  greenstone — Continued. 
Oceurrence  and  iliaraeter — Continued. 

^Microscopic  characters 150 

Texture I.5I 

Schistose  greenstones 153 

General  characters 153 

Origin 1 54 

Contact  metaniorphisni 155 

Contact  effect  of  granite  on  the  greenstone 155 

Mineralogic  composition  of  the  metamorphosed  rocks 157 

Contact  effect  of  gabbro  on  greenstone 161 

Relation  to  adjacent  formations 162 

Economic  value 163 

Interesting  localities 165 

Possible  tuffs  associated  with  the  greenstones 166 

Evidences  of  volcanic  character 166 

Metamorphism  of  the  greenstones 169 

Section  III.  Soudan  formation 172 

Occurrence  and  character 172 

Distribution 172 

Exposures I73 

Topography 175 

Structure I75 

Petrographic  characters 179 

Macroscopic  characters  of  the  fragmental  portion  of  the  Soudan  formation 179 

Microscopic  characters  of  the  fragmental  portion  of  the  Soudan  formation 180 

Macroscopic  characters  of  the  iron-bearing  formation  proper 181 

The  iron  ores , 183 

^Microscopic  characters  of  the  iron-bearing  formation  proper 185 

Origin 188 

Eelations  to  adjacent  formations 191 

Relations  to  the  Ely  greenstones 191 

Resume  of  relations  to  Ely  greenstones 198 

Eelations  to  the  Archean  acid  intrusives 199 

Relations  to  the  overlying  sediments ■  199 

Relations  to  basic  eruptives 200 

Age 200 

Thickness 200 

Interesting  localities 200 

Intricate  foldmg  of  the  Soudan  formation 210 

elastics  associated  with  the  iron-bearing  formation 212 

The  iron-ore  deposits 213 

Historical  sketch  213 

Ore  horizons 215 

The  Ely  iron-ore  deposits 216 

Deposits  occurring  at  the  bottom  of  the  iron  formation 216 

The  Tower  and  Soudan  deposits 222 

Deposits  occurring  at  bottom  of  the  iron  formation 223 

Deposits  occurring  within  the  iron  formation 224 

Origin  of  the  ore  deposits 227 

Methods  of  mining  in  the  Vermilion  district 234 

Prodncticin  and  shipments  of  iron  ore  from  the  Vermilion  district 241 

Prospect  ing 243 


CONTENTS.  7 

Chapter  III. — The  Akchean — Continued.  I'age. 

Section  IV.  Granites 246 

General  statement ""**' 

Age  of  the  acid  intrusives 2^" 

Granite  of  Vermilion  Lake '- 247 

Distribution,  exposures,  and  topography 247 

DistriVjution 247 

Exposures 247 

Topography 248 

Petrographic  characters 248 

^Macroscopic  characters 248 

Microscopic  characters 250 

Folding 250 

Structural  features  and  metamorphism 251 

Sericite-schists 253 

Chlorite-schists 253 

Schistose  granites  and  schists  derived  from  granites 253 

Relations  to  adjacent  formations 254 

Eelations  to  Lower  Huronian  series ; -  -  -  254 

Interrelations  of  granites  of  Vermilion  Lake '- 254 

Interesting  localities 255 

Localities  showing  relation  between  granite  of  Vermilion  Lake  and 

Ely  greenstone 25o 

Relations  of  the  acid  intrusives  to  the  Soudan  formation 256 

Relations  of  the  different  varieties  of  the  acid  intrusives  of  Vermilion 

Lake  to  one  another 257 

Granites  of  Trout,  Burntside,  and  Basswood  lakes 258 

Distribution,  exposures,  and  topography 258 

Distribution 258 

Exposures 258 

Topography 2.59 

Petrographic  characters - 2o9 

Macroscopic  characters 259 

Microscopic  characters 260 

Relations  to  adjacent  formations r 260 

Relations  to  Ely  greenstone 260 

Relations  to  other  intrusive  rocks 261 

Age 261 

Folding 262 

Interesting  localities 262 

Granite  between  Moose  Lake  and  Kawishiwi  River 263 

Distribution  and  exposures - 263 

Distribution 263 

Exposures 264 

Petrographic  characters 264 

Relations  to  adjacent  formations 264 

Relations  to  Archean 264 

Relations  to  Lower  Huronian 265 

Relations  to  Keweenawan 265 

Granite  of  Saganaga  Lake 265 

Distribution,  exposures,  and  topography 266 

Exposures -"" 

Distribution 266 

Topography -  266 


8  CONTENTS. 

Chapter  III. — The  Archean — Continued.  Page. 
Section  IV.  Granites — Continued. 

Granite  of  Saganaga  Lake — Continued. 

Petrograpliic  characters 266 

Macroscopic  characters 266 

Microscopic  characters 267 

Eelations  to  adjacent  formations 268 

Relations  to  Ely  greenstone 268 

Relations  to  Lower  Huronian  sediments 268 

Metamorphic  effects  of  the  granite  of  Saganaga  Lake 273 

Interesting  localities 273 

Chapter  IV. — The  Lower  Huronian 275 

Section  I.  Sedimentary  rocks 275 

Occurrence  and  subdivisions 275 

Vermilion  Lake  area 277 

Distribution,  exposures,  and  topography 277 

Distribution 277 

Exposures 278 

Topography 278 

Structure 278 

Relations  of  the  Ogishke  conglomerate  and  Knife  Lake  slate 281 

Relations  to  adjacent  formations 281 

■Relations  to  Archean 281 

Relations  to  Giants  Range  granite 283 

Relations  to  basic  dikes 283 

Age 284 

Ogishke  conglomerate 28-1 

Petrograpliic  characters 28-1: 

Macroscopic  characters 284 

Origin  of  the  conglomerates 285 

Thickness 286 

Interesting  localities 287 

Knife  Lake  slates 293 

Petrographic  characters 293 

Thickness 295 

Interesting  localities 295 

Knife  Lake  area 297 

Subdivisions 297 

Distribution,  exjiosures,  and  topography 297 

Distribution 297 

Exjiosures 298 

Topography 299 

Structure 299 

Relations 303 

Relations  of  the  sedimentary  members  of  the  series  to  one  another 303 

Relations  of  Lower  Huronian  sediments  to  adjacent  formations 303 

Relations  to  the  Archean 303 

Relations  to  Ely  greenstone 303 

Relati<ins  to  the  Soudan  formation oO-l 

Relations  to  the  granite  of  Saganaga  Lake 305 

Relations  to  i^ower  Huronian 305 

Relations  to  the  Giants  Range,  Snowbank,  and  Cacaquabic  granites, 

and  various  dikes  of  granites  and  granite-porphyry 305 

Relations  to  certain  basic  and  intermediate  dikes  of  Lower  Huronian 

age 306 

Relations  to  the  Upper  Huronian  series 306 


CONTENTS.  9 

Chapter  IV. — The  Lower  Hueonian — Continued.  Page. 
Section  I.  Sedimentary  rocks — Continued. 
Knife  Lake  area — Continued. 
Relations — Continued. 

Relations  of  Lower  Huronian  sediments  to  adjacent  formations — Continued. 

Relations  to  the  Keweenawan 307 

Relations  to  the  Keweenawan  gabl>ro 307 

Relations  to  basic  dikes 307 

Age 308 

Ogishke  conglomerate 309 

Petrographic  characters 309 

Macroscopic  characters 309 

Microscopic  characters 312 

Metamorphism 313 

Thickness 317 

Interesting  localities 317 

Agawa  formation  (iron  bearing) 324 

Distribution  and  exposures 324 

Distribution 325 

Exposures 326 

Structure 326 

Petrographic  characters 327 

Origin 329 

Relations  to  other  formations 329 

Age 330 

Thickness 330 

Interesting  localities 330 

Knife  Lake  slates 335 

Petrographic  characters 336 

Macroscopic  character 336 

Microscopic  character , 337 

Metamorphism 340 

Contact  effect  of  the  granite 340 

Contact  effect  of  the  gabbro ; 342 

Petrographic  characters  of  the  metamorphosed  slates 344 

Microscopic  characters 344 

Thickness 346 

Interesting  localities 347 

Structure  and  relations  of  the  slates 347 

Contact  metamorphism  of  the  slates 349 

Section  II.  Acid  and  basic  intrusives 352 

Introduction 352 

Giants  Range  granite 353 

Distribution,  exposures,  and  topography , 353 

Distribution 353 

Exposures : 354 

Topography 354 

Petrographic  characters 354 

Macroscopic  characters 354 

Relations  to  adjacent  formations 356 

Relations  to  Ely  greenstone 356 

Relations  to  Soudan  formation 356 

Relations  to  the  Lower  Huronian  sediments 356 

Relations  to  the  gabbro 357 

Age 358 


1 0  CO^'TENTS. 

Chapter  IV. — The  Lower  HrROXiAX — Continued.  Page. 

Section  11.  Acid  and  basic  intrusives— Continued. 
Giants  Range  granite — Continued. 

Folding 35S 

iletamorphic  action 359 

Interesting  localities 359 

Relations  to  the  Ely  greenstone 359 

Relation  to  Soudan  formation 359 

iletamorphism  caused  by  granite 359 

Snowbank  granite : 361 

Distribution,  exposiires,  and  topography • 361 

Petrographic  characters 361 

Relations  to  adjacent  formations 363 

Relations  to  the  Lower  Huronian 363 

Relations  to  the  Keweenawan  gabbro 363 

Interesting  localities - 364 

Cacaquabic  granite 364 

Distribution,  exposures,  and  topography 365 

Petrograjihic  character 365 

Relations  to  adjacent  formations 368 

Relations  to  the  Lower  Huronian 368 

Relations  to-  the  gabbro 368 

Age - 369 

Interesting  localities 369 

Various  acid  dikes 369 

Distribution 369 

Petrographic  characters 370 

Macroscopic  characters 370 

Microscopic  charactere 370 

Relations  to  adjacent  formations 370 

Basic  and  intermediate  intrusives 371 

Intei'esting  localities 373 

Chapter  V. — The  Upper  Hcroxiax  (Asimikie) 374 

Section  I.  Gunflint  formation 374 

Distribution,  exposures,  and  topography 375 

Distribution 375 

Exposures 376 

Topography 376 

Structure - 376 

Petrographic  characters 377 

Relation  to  other  formations 387 

Relations  to  the  Lower  Huronian  series — Ogishkeconglomerateand  Knife  Lakeslates.  388 

Relations  to  the  Keweenawan  (Duluth)  gabbro 389 

Relations  to  basic  dikes 390 

Thickness 390 

Section  II.  Rove  slate 390 

Distribution,  exposures,  and  topography 391 

Distribution 391 

Exposures 391 

Topography 391 

Structure 392 

Petrographic  characters 392 

Microscopic  characters 393 

Contact  metamorphism 393 


CONTENTS.  11 

Chapter  V. — The  Upper  Hi'RONian  (Aximikie)— Continued.  Page. 
Section  II.  Eove  slate — Continued. 

Eelation  to  adjacent  formations 395 

Relations  to  the  Keweenawan  dolerite 395 

Relations  to  the  Keweenawan  (Duluth)  gabbro 395 

Age 395 

.     Thickness - 395 

Chapter  VI. — The  Keweenawan 397 

Section  I.  Duluth  gabbro  and  Logan  sills 397 

Distribution,  exposures,  and  topography 398 

Distribution 398 

Exposures 399 

Topography 399 

Petrographic  characters  of  the  gabbro 401 

Macroscopic  characters 401 

Microscopic  characters 404 

Petrographic  characters  of  the  Logan  sills 405 

Macroscopic  characters 405 

Microscopic  characters 406 

Constituents 406 

Relations  of  the  gabbro  to  adjacent  formations 406 

Relations  to  the  Ely  greenstone 406 

Relations  to  the  Lower  Huronian  sediments 407 

Relations  to  the  Giants  Range  granite 407 

Relations  to  the  Snowbank  and  Cacaquabic  granites '  407 

Relations  to  the  Upper  Huronian  sediments 407 

Relations  to  the  Keweenawan 407 

Relations  of  the  Logan  sills  to  adjacent  formations 408 

Relations  to  the  Upper  Huronian  ( Animikie) 408 

Relations  to  the  Keweenawan 410 

Relations  of  the  gabbro  to  the  Logan  sills 410 

Conclusions  as  to  age  and  relation  of  the  gabbro  and  sills --  417 

Metamorphic  effect  of  gabbro  and  sills 418 

Archean  (Ely)  greenstone 418 

Lower  Huronian • 419 

Upper  Huronian  (Gunflint  formation) 419 

Rove  slates 419 

Endomorphic  action 419 

Iron-oxide  bodies  in  the  gabbro 420 

Character,  occurrence,  and  origin  ot  the  iron-oxide  bodies 420 

Section  II.  Acid  dikes  younger  than  the  Duluth  gabbro 422 

Section  III.  Basic  intrusives  younger  than  the  Duluth  gabbro 422 

Petrographic  characters 423 

Macroscopic  characters • 423 

Microscopic  characters 423 

Relations  to  adjacent  formations,  and  age 424 

Metamorphic  effects 424 

Chapter  VII. — The  drift 425 

Chapter  VIII. — Topoc;raphy  of  the  district  in  its  relations  to  geologic  structure 431 

Chapter  IX. — Geologic  history  of  the  Vermilion  district 437 

Index 449 


ILLUSTRATIONS. 


Paee. 
Plate  1.  Colored  map  showing  disti-ibution  of  pre-Cambriaii  and  other  rocks   in   the  Lake 
Snperior  region,  and  the  geographic  relations  of  the  Vermilion  district  of  Minnesota 

to  the  other  iron-bearing  regions  of  Lake  Superior 32 

II.  General  geologic  map  of  the  Vermilion  district 34 

III.  .-1  and  B,  Spherulitic  texture  in  the  greenstones 142 

IV.  A,  Ellipsoidal  parting  in  greenstone;  B,  EUipsoidally  parted  greenstone,    showing 

spherulitic  development 146 

V.  A,  Amygdaloidal  greenstone   (metabasalt);    B,  Magnetitic  chert,  showing  possible 

lines  of  false  bedding 168 

VI.  A,  Folded  jasper  about  one-half  mile  east  of  Jasper  Peak;  B,  Contorted  jasper  and  ore 
in  the  NE.  i  sec.  2.5,  T.  63  N.,  K.  12  AV.;  C,  A  breccia  of  jasper  and  chert  fragments 

in  greenstone  matrix 176 

A^II.  Folded  jasper  and  slate,  showing  slaty  cleavage  developed  in  slate  bands 178 

VIII.  Panoramic  view  of  the  ore  basin  north  of  Ely,  showing  shafts  of  the  Chandler  and  the 

Pioneer  mines 216 

IX.  A,  View  of  main   level   timbering,  Jlinnesota  mine;  B,  Filling  system,   Minnesota 

Iron  Company,  Minnesota,  with  chute  for  discharging  refuse  from  upper  levels 236 

X.  A,  View  showing  method  of  loading  cars;  B,  View  of  main  drift  which  has  begun  to 

cave.  Chandler  mine 238 

XI.  Scrammers  in  mine  using  caving  system 240 

XII.  A,  Photomicrograph,  showdng  granules  in  Gunflint  formation;  B,  Photomicrograph, 

showing  details  of  granules 382 

XIII.  .4,  View  of  sawtooth  hills  of  Rove  slate  capped  with  dolerite  sills,  at  the  northeast 
end  of  Rose  Lake,  international  boundary;  B,  View  on  an  island  in  Burntside  Lake, 
showing  granite  of  Burntside  Lake  cutting  amphibole-schists,  metamorphosed  Ely 

greenstone 392 

Fig.    1.  Reproduction  of  sketch  by  A.  Winchell  showing  the  intricate  relationship  between  the 

granite  of  Burntside  Lake  and  the  amphibole-schists lo8 

2.  Reproduction  of  sketch  by  A.  "Winchell  showing  the  intricate  relationship  between  the 

granite  of  Burntside  Lake  and  the  amphibole-schists 159 

3.  Sketch  showing  Soudan  formation  infolded  in  Ely  greenstone,  both  cut  by  Keweenawan 

dolerite  dike , , 205 

4.  Illustration  showing  distribution  and  relations  of  Ely  greenstone,  Soudan  formation, 

and  Ogishke  conglomerate  south  of  Moose  Lake 206 

5.  Sketch  showing  association  and  relations  of  Ely  greenstone,  Soudan  formation,  and 

Ogishke  conglomerate 207 

6.  Sketch  showing  relations  between  Soudan  formation  and  Ely  greenstone  on  Otter  Track 

Lake - 208 

7.  Sketch  made  in  the  field,  showing  relations  of  Soudan  formation  and  Ely  greenstone  on 

Jasper  Lake 209 

•8.  Vertical  section  across  Chandler  ore  body  along  line  E-F  of  fig.'  9 216 

13 


14  ILLUSTRATIONS. 

Page. 

Fill.    9.  Horizontal  section  througli  fourth  and  sixth  levels  of  the  Chandler  mine 217 

1 0.  A'ertical  east-west  section  through  the  Chandler  mine 218 

11.  Vertical  .section  through  the  Chandler  ore  basin  along  the  line  A-B  of  fig.  9 219 

12.  Reproduction  of  sketch  showing  replacement  of  jasper  by  iron  ore 231 

13.  Cross  section  at  Ko.  8  shaft,  Soudan,  Minn ' 235 

14.  Horizontal  section  through  the  fifth  level  of  No.  8  shaft,  Soudan,  Minn 236 

15.  Longitudinal  section  through  Soudan  mine 237 

16.  Cross  section  of  Soudan  mine  showing  raise 238 

17.  Detail  geologic  map  showing  exposures  in  a  small  area  on  West  Gull  Lake 272 

18.  Detail  map  of  east  end  of  Ely  Island 282 

19.  Sketch  showing  intricate  relationship  of  granite-porphyry  and  overlying  Ogishke  con- 

glomerate on  Ely  Island,  Vermilion  Lake 290 

20.  Sketch  showing  relationship  of  Ely  greenstone  and  overlying  Ogishke  conglomerate  on 

island  in  Ogishke  Muncie  Lake ;  22 

21.  Diagrammatic  section  across  the  west  end  of  Gunfiint  Lake,  illustrating  the  character- 

istic topograph}'  of  the  Rove  slate  area 392 

22.  Large-scale  section  through  the  Rove  slates  with  intercalated  Logan  sills 400 

23.  Sketch  map  showing  the  distribution  of  the  Vermilion  moraine  in  the  Vermilion  dis 

trict  of  Minnesota  ..' --  427 


ATLAS  SHEETS. 


Sheet. 

Title I 

Contents II 

Legend  and  key  map Ill 

Topographic  map  of  part  of  tlie  Tower  quadrangle IV 

Topographic  map  of  part  of  the  Soudan  quadrangle Y 

Topographic  map  of  part  of  the  El}'  quadrangle VI 

Topographic  map  of  parts  of  the  Basswood  and  Fall  Lake  quadrangles VII 

Topographic  map  of  parts  of  the  Ensign  and. Snowbank  Lake  quadrangles VIII 

Topographic  map  of  part  of  the  Knife  Lake  quadrangle IX 

Topographic  map  of  part  of  the  Gunflint  Lake  quadrangle X 

Geologic  map  of  part  of  the  Tower  quadrangle,  with  structure  sections XI 

Geologic  map  of  part  of  the  Soudan  quadrangle,  with  structure  sections XII 

Geologic  map  of  j)art  of  the  Ely  quadrangle,  with  structure  sections XIII 

Geologic  map  of  part  of  the  Basswood  and  Fall  Lake  quadrangles,  with  structure  sections XIV 

Geologic  map  of  part  of  the  Ensign  and  Snowbank  Lake  quadrangles,  with  structure  sections.  XV 

Geologic  map  of  part  of  the  Knife  Lake  quadrangle,  with  structure  sections XVI 

Geologic  map  of  part  of  the  Gunflint  Lake  quadrangle,  with  structure  sections XVII 

Detail  map  of  a  portion  of  Tps.  61-62  N. ,  E.  15  W. ,  Minnesota XVIII 

Detail  map  of  a  portion  of  Tps.  62-63  X.,  R.  U  W.,  Minnesota XIX 

Detail  map  of  a  portion  of  Tps.  62-63  N.,  E.  13  W.,  Minnesota XX 

Detail  map  of  portions  of  Tps.  62-63  N.,  E.  12  W.,  Minnesota XXI 

Detail  map  of  a  portion  of  T.  63  N.,  E.  11  W.,  Minnesota XXII 

Detail  map  of  Tower  and  Lee  hills XXIII 

Detail  map  of  Soudan  Hill XXIV 

Detail  map  of  laoint  south  of  Mud  Creek  Bay,  Vermilion  Lake,  ilinnesota XXV 

Detail  map  showing  actual  exposures  in  NE.  \  sec.  25,  T.  63  X.,  E.  12  W.,  with  structure  sec- 
tions   XXVI 

15 


LETTER  OF  TRANSMITTAL. 


Department  of  the  Interior, 

United  States  Geological  Survey, 

Washington,  D.  C,  June  30,  1902. 

Sir:  I  transmit  herewith  the  manuscript,  illustrations,  and  atlas  of 
a  monograph  on  the  Vermilion  iron-bearing-  district  of  Minnesota,  b)^ 
J.  Morgan  Clements. 

This  monograph  is  the  fifth  one  of  a  series  of  six  which  treat  of  the 
iron-bearing  districts  of  the  Lake  Superior  region.  The  monographs  on 
the  Penokee,  Marquette,  Crystal  Falls,  and  Mesabi  districts  have  already 
been  published.  The  last  of  the  series  is  a  monograph  on  the  Menominee 
district,  by  W.  S.  Bayley.  There  has  also  appeared  a  monograph  on  ihe 
copper-bearing  rocks,  by  R.  D.  Irving.  It  is  planned  to  close  the  work  of 
the  United  States  Geological  Survey  in  the  Lake  Superior  country  by  a 
final  monograph  on  the  Lake  Superior  region  as  a  whole. 

This  report  contains  the  first  series  of  detailed  maps  of  the  Vermilion 
district.  This  region  is  one  in  which  the  rocks  of  Archean  age  contain 
economic  deposits.  The  geologic  mapping  of  the  intricately  folded  Archean 
rocks  has  been  a  task  of  very  great  labor,  requiring  the  full  field  seasons  of 
several  men  from  1897  to  1899,  inclusive,  and  a  part  of  that  of  1900.  The 
area  mapped  in  detail  is  about  1,000  square  miles. 

The  topographic  work  for  the  report  was  done  by  Robert  Muldrow 
and  E.  C.  Bebb,  with  various  assistants.  The  geologic  mapping  has' 
been  done  more  largely  by  J.  Morgan  Clements  than  anyone  else,  but 
W.  S.  Bayley  and  C.  K.  Leitli  have  also  done  a  large  amount  of  areal 
mapping,  and  W.  N.  Merriaim  has  made  a  number  of  large-scale,  detailed 
plats  of  certain  areas  having  exceptional  economic  importance.     My  own 

MON  XLV— 03 2  17 


18  LETTER  OF  TRANSMITTAL. 

part  of  the  work  lias  been  a  general  super-vision  of  the  survey.  This 
has  involved  frequent  trips  into  the  district,  made  in  order  to  assist  in 
solving  the  general  structural  jDroblems. 

In  our  work  on  the  Vermilion  district  we  have  had  the  willing  help  of 
the  officers  in  charge  of  the  mines,  and  we  are  very  greatly  indebted  to 
them  for  their  assistance.  In  this  connection  we  would  especially  mention 
Mr.  D.  H.  Bacon,  who  formerly  was  president  of  the  Minnesota  Iron  Com- 
pany, and  Mr.  T.  F.  Cole,  now  president  and  general  manager  of  the 
Minnesota  Iron  Company. 

Very  respectfully,  C.  R.  Van  Hise, 

Geolofjist  in  Charge. 
Hon.  Charles  D.  "Walcott, 

Director  of  United  States  Geological  Survey. 


OUTLINE  OF  MONOGRAPH 


Chapter  I.  The  Vermilion  iron-bearing  district  of  Minnesota  resembles  the 
other  iron-bearing  districts  of  the  Lake  Superior  region  in  that  the  rocks  are  of 
very  great  geologic  age.  Its  economic  importance  has  been  known  for  a  rela- 
tively short  time,  the  first  published  statement  of  the  occurrence  of  iron  ore  in  this 
district  having  been  made  in  1850.  A  brief  statement  is  made  of  the  geologic  work 
previously  done  in  this  district,  including  the  names  of  the  geologists  hj  whom  it 
was  done,  and  the  scope  of  the  paper  is  then  outlined.  The  territory  included  in 
the  Vermilion  iron-bearing  district  lies  in  the  extreme  northeastern  portion  of 
Minnesota,  including  portions  of  St.  Louis,  Lake,  and  Cook  counties.  The  district 
has  an  area  of  approximately  1,000  square  miles.  It  is  a  narrow  belt  trending  east- 
northeast,  which  ranges  from  2  to  IS  miles  in  width,  and  has  a  length  of  somewhat 
over  100  miles,  extending  from  the  west  end  of  Vermilion  Lake  to  Gunflint  Lake, 
on  the  international  boundarj'.  From  a  topographic  standpoint  the  Vermilion 
district  is  divisible  into  four  areas,  each  of  which  is  characterized  b}'  a  fairly 
distinct  type  of  topographic  development.  The  first  of  these  areas  described  is 
the  one  including  the  Giants  range,  the  most  prominent  topographic  feature  of  the 
Vermilion  district.  The  range  reaches  an  extreme  height  of  2,120  feet  above  sea 
level,  but  in  general  is  not  a  very  prominent  feature  throughout  its  extent.  It 
forms  the  backbone  of  the  district,  extending  across  it  in  a  northeast  direction  and 
dividing  it  into  unequal  areas.  The  second  area  described  lies  north  of  the  Giants 
range  and  includes  all  of  the  areas  underlain  by  the  most  important  iron-bearing 
formation.  This  area  is  characterized  by  ridges  trending  N.  70°-80'^  E.,  with 
intervening  valleys,  the  larger  ones  usually  occupied  by  streams  or  lakes.  In  this 
area  the  topography  is  very  rugged,  but  the  range  in  altitude  is  not  great.  The 
third  area  described  is  the  high  plateau  country  Ijnng  southeast  of  the  Giants  range 
and  underlain  by  gabbro.  The  fourth  is  a  small  triangular  area  at  the  extreme  east 
end  of  the  district,  lying  between  the  Giants  range  on  the  north  and  the  gabbro 
plateau  on  the  south.  In  this  area  a  rather  peculiar  topography-  is  developed.  The 
hills  have  abrupt  north  escarpments  and  gentle  south  slopes.  These  ridges  lie  the 
one  south  of  the  other,  and  present  in  profile  the  appearance  of  a  series  of  saw 
teeth;  hence  they  are  commonly  spoken  of  as  "'sawtooth  mountains."     That  the 

drainage  system  is  immature  is  shown  by  the  abundance  of  lakes  in  the  district,  by 

19 


20  OUTLINE  OF  MONOGRAPH. 

the  absence  of  large  streams,  by  the  fact  that  the  siiiull  and  short  streams  which 
do  exist  serve  mei-ely  to  connect  the  lakes  into  strings,  and  by  the  fact  that  these 
streams  are  frecjuently  interrupted  in  their  courses  by  rapids  and  falls.  Large 
swamps  still  further  emphasize  this  imperfect  drainage.  The  lakes  and  streams 
that  feed  and  drain  them  belong  to  the  large  basins  of  the  St.  Lawrence  Hirer 
and  Hudson  Bay.  The  area  belonging  to  the  St.  Lawrence  drainage  basin  is  very 
small  and  is  drained  by  only  one  small  stream,  representing  the  headwaters  of  the 
Embarrass  River,  which  flows  south  and  linalh'  empties  into  Lake  Superior.  By 
far  the  greater  part  of  the  district  belongs  to  the  Hudson  Bay  drainage  system. 
All  the  waters  of  this  sj'stem  flow  north  and  west,  and  are  collected  in  Rainy  Lake. 
The  streams  are  short,  narrow,  and  shallow,  and  form  merelj'  the  connections 
between  the  numerous  lakes.  The  lakes  are  far  more  abundant  in  the  eastern  than 
in  the  western  poition  of  the  district.  They  lie  in  basins  which  in  general  trend 
east-northeast  and  constitute  the  main  routes  of  travel  within  the  district.  Most 
of  the  lakes  have  had  a  mixed  mode  of  origin,  owing  their  existence  to  pre- 
Glacial  erosion,  which  scooped  out  deep  valleys,  and  then  to  the  drift,  which  left 
dams  across  these  valley's  at  intervals  along  their  lengths,  forming  the  strings  of 
lakes  that  we  now  find.  Other  lakes  appear  to  owe  their  present  location  and 
existence  solelj'  to  glacial  action.  Thej-  are  depre.ssions  in  the  drift  which  have 
been  filled  hy  water.  -Rock  exposures  are  numerous,  especially  in  the  immediate 
vicinity  of  the  lakes,  and  are  particularly  abundant  in  the  eastern  part  of  the 
district.  Only  a  small  area  in  the  district  is  wooded  with  old  forests.  A  very 
large  part  of  the  district,  particularly  the  eastern  portion,  has  been  burned  over 
repeatedly,  and  here  almost  all  growth  is  wanting,  or  there  is  but  a  meager  second 
growth  of  small  timber  present.  The  district  is  well  supplied  with  fish  and  game. 
There  are  only  four  towns  in  the  district — Tower.  Soudan,  Ely,  and  Winten.  Tower 
is  the  oldest:  it  was  settled  in  1882,  and  has  1,366  inhabitants,  according  to  the 
Twelfth  Census.  Elv.  the  largest  place,  has  3,717  inhabitants.  The  first  three 
places  named  depend  almost  altogether  upon  the  mining  industry.  •  Winten  is  a 
small  village  whose  existence  is  dependent  upon  two  sawmills  which  are  rapidly 
cutting  the  timber  remaining  in  the  district.  There  is  one  Indian  reservation  in  the 
district,  that  of  the  Bois  Fort  band  of  the  Chippewa  Indians,  on  Sucker  Point, 
where  there  are  reported  to  be  808  Indians  living.  As  a  matter  of  fact,  there  are 
rai'ely  more  than  7.5  or  100  Indians  actually  upon  the  reservation,  at  least  during 
the  summer,  the  remainder  being  scattered  through  the  surrounding  country.  They 
are  not  progressive,  and  while  apt  in  the  acquirement  of  the  vices  of  civilization, 
do  not  appear  to  be  willing  to  bear  any  of  its  burdens. 

Chaiter  II.  In  this  chapter  there  is  given  a  1)rief  statement  of  the  main  canoe 
routes  of  the  district.  The.  methods  of  travel  are  described  by  quotations  from  the 
journals  of  the  fur  traders,  as  this  shows  the  conditions  existing  when  tht>  country 
was  being  opened  up.  The  methods  of  tra\el  are  now  essentially  the  same,  by 
canoe,  although  manv  of  the  old  customs  have  died  out.     The  remainder  of  the 


OUTLINE  OF  MONOGRAPH.  21 

chapter  is  devoted  to  abstracts  of  articles  dealing  with  the  geology  of  the  district. 
In  these  abstracts  the  authors  have  been  quoted  very  freely.  From  a  perusal  of  this 
chapter  one  can  obtain  an  idea  of  the  growth  of  knowledge  of  the  geology  of  the 
district,  which  is  comparativelj^  difficult  of  access. 

Chapter  III.    This  chapter  deals  with  the  Arphean. 

Section  I  gives  the  definition  and  subdivisions  of  the  Archaen.  As  a  result  of 
studies  made  largely  in  this  district  it  was  found  necessary  to  modify  the  definition 
of  the  Archean  so  as  to  include  within  it  some  small  quantities  of  sediments.  The 
Ai'chean  of  the  Vermilion  district  is  divided  into  thi'ee  formations,  as  follows,  given 
from  the  base  up:  The  Ely  greenstone,  the  iron-bearing  Soudan  formation,  and  the 
granites  of  Vermilion,  Trout,  Burntside,  Basswood,  and  Saganaga  lakes. 

In  Section  II  the  Elj'  greenstone  is  described.  This  formation  consists  of  basic 
to  intermediate  igneous  rocks,  and  is  the  lowest  member  of  the  geologic  column. 
These  greenstones  are  very  widely  distributed  and  occur  normally  in  anticlinal  areas, 
as  is  shown  by  the  distribution  of  the  overlying  sedimentaries.  A  petrographic 
study  of  the  greenstones  shows  that  they  were  originally  rocks  corresponding  in 
character  to  intermediate  andesites  and  basic  basalts.  They  have  been  extremely 
altered,  but  retain  in  many  cases  in  striking  perfection  the  original  structures,  such 
as  ellipsoidal  parting  and  spherulitic  and  amygdaloidal  structures.  A  study  of  their 
various  textures  and  structures  shows  that  these  greenstones  are  unquestionaljl}^  of 
igneous  origin,  and  are  largely  of  volcanic  character.  With  the  volcanics  there  are 
associated,  of  course,  some  intrusives  of  essentially  the  same  age.  These  have  been 
subjected  not  only  to  the  ordinar}-  processes  of  alteration  that  have  metamorphosed 
the  greenstones,  but  have  been  strongly  compressed  and  in  many  cases  have  become 
schistose.  Actual  green  schists,  however,  are  very  subordinate  in  quantity.  The 
greenstones  have  also  been  strongly  affected  by  the  contact  metamorphism  due  to 
the  intrusion  of  great  granite  masses.  As  a  result  of  this  intrusion  there  have  been 
produced  from  the  greenstones  amphibole-schists,  which  form  a  marginal  facies  of 
the  greenstones,  lying  between  them  and  the  adjacent  granites.  The  gi'eenstones 
have  also  been  metamorphosed  by  the  Duluth  gabbro  of  Keweenawan  age,  and 
granular  rocks  have  thus  been  produced  which  in  most  cases  show  the  original 
textures  of  the  greenstones,  but  contain  also  a  development  of  fresh  biotite,  hyper- 
sthene,  brown-green  hornblende,  and  magnetite.  These  greenstones  have  very 
slight  value  at  present,  although  they  make  good  road  material. 

In  Section  III  the  iron-bearing  Soudan  formation  of  the  Archean  is  treated.  The 
iron  formation  is  widel}^  distributed  in  the  western  part  of  the  district,  but  is 
practicall}'  wanting  in  the  eastern  half.  Where  it  occurs  it  is  found  mosth'  in 
narrow  belts,  which  consist  largelj^  of  greenstone  so  intimately  associated  with  the 
iron  formation  that  it  has  been  impossiVjle  to  separate  them  on  the  map.  In  spite 
of  the  resistant  character  of  the  rocks  constituting  the  formation,  exposures  are  not 
verj'  good,  and  it  has  been  difficult  to  trace  out  continuous  belts.  The  Soudan 
being  the  oldest  sedimentary  formation  in  the  district  has  been  subjected  to  all  the 


22  OUTLINE  OF  MONOGKAPH. 

oroo-eiiic  movements  that  have  occurred  since  its  deposition.  In  consequence  of  thi? 
its  rocks  hu\e  been  most  intricately  folded.  Where  it  is  exposed  most  prominenth" 
it  forms  anticlines,  although  upon  these  are  numerous  minor  rolls,  giving-  folds  with 
Steep  pitches.  The  formation  consists  of  (1)  a  very  subordinate  fragmental  portion 
made  up  of  some  conglomerate,  clearly  recognizable  as  having  been  derived  from 
the  underlying  greenstones,  grading  up  into  sediments  of  finer  character;  and  (2) 
lying  above  this  fragmental  portion,  the  iron-bearing  formation  proper,  which 
consists  of  siliceous  rocks,  largely  white  cherts— though  varying  in  color  from  white, 
green,  yellow,  and  purplish  to  black — with  red  jasper  and  carbonate-bearing  chert, 
griinerite-magnetite-schist,  hematite,  magnetite,  and  small  quantities  of  pyrite. 
These  various  rocks  occur  in  bands  of  varying  thickness.  Where  banded  they 
rarely  exceed  a  thickness  of  5  or  6  inches.  The  hematite  occurs  in  certain  places 
in  masses  of  variable  size,  which  constitute  the  ore  deposits.  These  iron-bearing 
rocks  are  clearly  of  sedimentary  origin.  They  do  not  now  present  their  original 
characters,  but  are  presumed  to  have  been  derived  from  rocks  that  were  largely 
carbonate-liearing,  ferruginous  cherts.  The  relation  of  the  iron  formation  to  the 
adjacent  greenstones  is  clearly  that  of  a  sedimentary  overlying  an  igneous  series. 
The  few  basal  conglomerates  of  the  iron  formation  that  have  been  found  consist  of 
pebbles  derived  from  the  underlying  greenstone,  showing  conclusively  their  relation- 
ship. This  relationship  is  obscured,  however,  in  most  places,  by  the  absence  of 
the  conglomerates,  and  by  the  fact  that  the  iron  formation  has  been  very  closely 
infolded  in  the  greenstone.  In  consequence  of  the  extreme  folding  and  of  the 
impossibility  of  determining  different  horizons  in  the  iron  formation,  it  has  been 
impracticable  to  ascertain  its  thickness. 

The  first  published  statement  of  the  occurrence  of  iron  ore  in  the  Vermilion 
district  was  made  by  J.  G.  Norwood  in  1850.  After  a  brief  period  of  exploration 
for  gold  in  the  sixties  the  attention  of  explorers  was  turned  to  the  development  of 
the  iron  deposits.  As  the  result  of  this  development  a  railroad  was  built  to  Tower 
in  1884,  and  shipments  of  the  ore  began.  The  ores  are  extremely  hard,  massive, 
blue  hematites.  In  the  Chandler  mine  the  ores  have  been  brecciated,  but  the 
fragments  of  the  breccia  are  .still  the  hard  blue  hematite,  averaging  about  63.  T  per 
cent  iron,  0.05  per  cent  phosphorus,  -±.78  per  cent  silica,  and  5.5  per  cent  water. 

The  iron-ore  deposits  of  the  Vermilion  district  show  a  striking  analogy  wifh 
those  of  the  Marquette  district.  Like  them,  they  may  occur  in  two  positions  with 
respect  to  the  iron-bearing  formation.  They  are  found  first  at  the  bottom  of  this 
formation,  and  second  within  it,  the  ores  in  both  cases  being  the  same  in  character. 
The  largest  known  deposits  are  at  Ely.  These  are  typical  of  the  deposits  occurring 
at  the  base  of  the  formation.  They  are  found  at  the  bottom  of  a  closely  compressed 
.syncline  of  the  iron  formation  which  lies  in  the  relatively  impervious  greenstone. 
The  source  of  the  iron  was.  in  the  first  instance,  the  Ely  greenstone.  From  this 
it  was  removed  through  the  action  of  water  and  collected  in  the  Archean  sea  to 
form  the  sedimentary  deposits  of  the  .Soudun  formation.  After  tlH>  folding  of  the 
formation  this  disseminated  iron  was  carried  l>y  downward-percolating  waters  into 


OUTLINE  OF  MONOGRAPH.  23 

places  favorable  for  its  accumulation,  such  as  the  bottom  of  this  synclinal  trough, 
where  it  was  precipitated  by  oxygen-bearing  waters  coming  more  directly  from  the 
surface.     Pari  passu  with  this  precipitation  silica  was  removed,  affording  space  for 
the  accumulation  of  the  iron  to  form  the  ore  deposits  as  now  known.     The  Tower 
and  Soudan  deposits  differ  only  in  detail  from  the  Ely  deposit.     They  were  accu- 
mulated in  favorable  places  both  at  the  bottom  of  the  formation,  where  it  rests 
against  the  greenstone  in  which  it  is  infolded,  and  within  the  formation  in  basins 
formed  by  the  intrusion  and  subsequent  folding  of  igneous  rocks.     The  mode  of 
accumulation  in  these  is  the  same  as  that  briefly  outlined  for  the  Ely  deposits. 
The  methods  of  mining  in  the  Vermilion  district  are  briefly  described. 
In  Section  IV  are  described  certain  acid  intrusives  varying  from  fine-  to  coarse- 
grained granites,  and  from  porphyries  with  very  fine-grained  groundmass  to  granite- 
porphyries.     The  granites  are  known  from  the  topographic  features  with  which 
they  are  associated,  as  the  granites  of  Vermilion,  Trout,  Burntside,  and  Basswood 
lakes,  the  granite  between  Moose  Lake  and  the  Kawishiwi  River,  and  the  granite  of 
Saganaga  Lake.     All  of  these  rocks  are  younger  than  the  Ely  greenstone,  for  they 
occur  in  it  as  dikes.     A  number  of  these  dikes  are  found  also  in  the  iron-bearing 
Soudan  formation,  which  is  of  more  recent  origin  than  the  greater  part  of  the  Ely 
greenstone.     That  these  intrusives  are  older  than  the  Ogishke  conglomerate  (Lower 
Huronian),  which  succeeds  in  age  the  Soudan  formation,  is  shown  conclusively  by 
the  fact  that  pebbles  derived  from  them  occur  in  this  conglomerate.     The  general 
period  of  intrusion  of  all  of  these  acid  igneous  rocks  is  placed  between  the  time  of 
the  deposition  of  the  latest  sediments  of  the  Archean  and  that  of  the  deposition  of 
the  earliest  sediments  of  the  Lower  Huronian  series.     Some  were  perhaps  intruded 
near  the  beginning  of  this  interval,  others  probably  near  the  end,  but  it  is  now 
impossible  to  give  their  exact  ages.     In  the  portion  devoted  to  the  granites  of  the 
different  areas  the  various  intrusives  are  described  somewhat  in  detail.     Their  petro- 
graphic  characters  are   given  as  hornblende-  and  mica-granites,  and  the  various 
schistose  rocks  produced  from  them  are  described. 

Chapter  IV.  This  chapter  is  devoted  to  a  description  of  the  Lower  Huronian 
series.  In  Section  I  are  discussed  the  sedimentary  rocks  of  this  series,  which  have 
a  very  large  surface  extent  in  the  Vermilion  district.  They  are  present  in  two 
large  detached  areas,  one  of  which,  known  as  the  Vermilion  Lake  area,  extends  from 
the  western  limit  of  the  acea  mapped,  in  the  vicinity  of  Tower,  to  within  about  11 
miles  of  Ely  on  the  east.  The  second  area  begins  about  7  miles  west  of  Ely  and 
extends  eastward  to  the  eastern  limit  of  the  area  mapped.  This  is  known  as  the 
Knife  Lake  area.  The  rocks  of  these  two  areas,  although  of  slightly  different 
petrographic  character,  are  of  essentially  the  same  age.  At  the  base  of  the  series 
there  lies  a  great  conglomerate,  known  as  the  Ogishke  conglomerate.  The  relation 
of  this  conglomerate  to  the  formations  previously  described  is  conclusively  shown 
by  the  fact  that  it  consists  of  pebbles  and  finer  detritus  derived  from  the  Ely  green- 
stone, the  Soudan  formation,  and  the  various  acid  intrusives  already  mentioned. 
Above  this  conglomerate  in  the  eastern  portion  of  the  district  there  are  found  m  a 


24  OUTLINE  OF  MONOGRAPH. 

few  localities  small  masses  of  the  iron-bearing  Agawa  formation.  This  formation 
is  petrographically  the  same  as  the  Soudan  fonnation.  In  it,  however,  there  is  in 
places  a  development  of  the  carbonate-bearing^  facies.  No  iron  ores  have  been 
found  in  it,  and  it  is  of  so  small  a  surficial  extent,  and  so  thin,  that  no  large  iron-ore 
deposits  will  probablj'  ever  be  found  in  it  in  the  United  States  in  the  Vermilion 
disti'ict  proper.  A  reconnaissance  made  in  the  adjacent  portion  of  Ontario  indicates 
that  it  is  there  better  developed  than  on  the  United  States  side  of  the  border,  and  it 
may  possibly  contain  iron  deposits  in  this  area,  although  this  does  not  seem  to  be 
very  probable.  This  iron-bearing  formation  is  wanting  in  the  western  portion  of 
the  Vermilion  district. 

Overlying  the  Ogishke  conglomera,te  in  the  western  portion  of  the  district  and 
the  intervening  iron-bearing  Agawa  formation  where  present  in  the  eastern  portion 
of  the  district,  there  occurs  a  thick  series  of  slates  of  varying  character,  to  which 
the  name  Knife  Lake  slates  has  been  given.  These  slates  have  been  very  closelj' 
folded.  Owing  to  the  lack  of  well-defined  horizons  in  the  conglomerates  and  in  the 
slates  it  has  been  impossible  to  trace  out  the  structure  of  this  series  by  following 
key  rocks.  The  folding  has,  however,  been  proved  in  manj'  localities  by  a  study  of 
the  distribution  of  these  rocks.  The  relation  of  this  series  to  the  older  rocks  is 
shown  by  the  fact  that  it  consists  of  detritus  derived  from  these  older  rocks.  In 
three  large  areas  granites  which  are  younger  than  the  sediments  are  associated 
with  them.  These  granites  are  known  as  the  Giants  Range  granite,  the  Snowbank 
granite,  and  the  Cacacjuabic  gi'anite.  This  relationship  is  proved  by  the  fact  that 
these  granites  cut  through,  send  offshoots  into,  and  have  metamorphosed  the 
sediments.  As  a  result  of  this  metamorphism,  micaceous  conglomerates  in  which 
the  conglomeratic  structure  is  still  recognizable  have  been  produced  from  the 
Ogishke  conglomerate,  and  mica-schists  have  been  produced  from  the  Knife  Lake 
slates.  These  sediments  are  also  metamorphosed  by  the  Duluth  gabbro,  which  has 
changed  them  into  mica-schists.  Hence  the  gabbro  is  younger  than  the  sediments. 
In  addition  there  are  found  in  the  rocks  of  the  series  certain  basic  dikes  which  are 
similar  to  others  which  cut  the  Duluth  gabbi'o,  and  which  are  considered  to  be  of 
Keweenawan  age. 

In  Section  II  of  this  chapter  various  acid  intrusives  of  the  same  general 
character  petrographically,  and  of  the  same  geologic  age,  are  discussed.  They  are 
granites  and  granite-porphyries  whicli  occur  in  large  masses  and  in  dikes  penetrating 
the  surrounding  Lower  Huronian  sediments  and  other  adjacent  rocks.  From  their 
occurrence  in  the  vicinity  of  the  Giants  Range,  Snowbank  Lake,  and  CacaquaMc 
Lake  these  names  have  been  given  to  the  gTanites  occurring  in  these  areas, 
respectively.  There  is  included  also  a  description  of  some  acid  and  intermediate 
intrusives  of  the  same  age  as  the  large  masses  of  acid  intrusives.  The  Giants  Range 
granite  is  a  h()rnl)lende-mica-granite,  and  varies  from  very  tine-grained  rocks 
through  medium-grained  to  coarse-grained  rocks.  The  Snowbank  gi-anite  also 
varies  from  tine-  to  coarse-grained  forms,  with  medium-grainod  facies  as  the  most 


OUTLINE  OF  MONOGRAPH.  25 

abundant  type.  Cei'tain  porphyritic  facies  of  the  granite  also  occur.  This  granite 
varies  from  a  normal  mica-  and  hornblende-granite  to  an  augite-granite,  and  by 
loss  of  quartz  to  a  syenite.  The  Cacaquabic  granite  is  somewhat  more  interesting 
than  the  preceding  ones,  in  that  it  is  one  of  the  rather  exceptional  augite-soda-granites. 
The  main  mass  of  this  granite  is  developed  as  a  medium-grained  gra}^  or  pink  to  red 
granite,  whereas  on  the  periphery  of  the  granite  area  a  finer-grained  granite  and 
also  a  granite-porph^-rv  facies  of  the  rock  are  developed.  In  addition  to  this  there 
are  various  granite  and  granite-porphyry  dikes  whose  immediate  relationship  to  the 
granite  niassives  already  described  could  not  be  traced  in  the  field.  A  section  ia 
devoted  to  a  brief  description  of  certain  basic  and  intermediate  intrusives  of  doleritic 
and  lamprophyric  character,  which  bear  the  same  relations  to  the  various  adjacent 
formations  as  do  the  acid  rocks  previously  described. 

Chapter  V.  This  chapter  treats  of  the  Upper  Huronian  (Animikie)  series. 
This  series  is  found  in  the  extreme  eastern  portion  of  the  district,  where  it  underlies 
a  relatively  small  area.  It  is  known,  however,  to  have  enormous  development 
to  the  east,  immediately  beyond  the  limits  of  the  Vermilion  district,  and  also  to  the 
south-southwest,  in  the  adjacent  Mesabi  district.  This  Upper  Huronian  series  may 
be  readily  divided  into  two  facies  of  I'ocks  that  are  quite  different  petrographically. 
At  the  bottom  of  the  series  occurs  an  iron-bearing  formation  known  as  the  Gunllint 
formation.  Above  this  occurs  a  great  slate-graywacke  formation  to  which  the 
name  Rove  slate  has  been  given.  The  Gunflint  formation  is  correlated  with  the 
Biwabik  formation  of  the  Mesabi  district.  It  has  a  verj^  limited  development  in 
the  Vermilion  district,  and  its  most  interesting  phases  are  especially  well  developed 
in  the  vicinity  of  Akeley  Lake.  In  general  the  rocks  of  this  formation  have  a 
monoclinal  dip  to  the  south-southeast  at  a  low  angle,  but  variations  in  the  strike 
and  dip  indicate  clearly  that  the  structure  is  not  so  simple  as  it  appears  to  be. 
Minor  folds  have  been  traced.  Petrographically  the  rocks  of  the  Gunflint  formation 
are  peculiar.  Where  least  metamorphosed,  they  consist  of  thin  bands  of  nearly 
pure  chert  alternating  with  cherty  and  granular  quartzose  bands  containing  varying 
percentages  of  iron  carbonate,  bands  of  jasper,  magnetitic  chert,  and  other  bands 
consisting  of  quartz  as  a  basis  with  actinolite  and  griinerite  crystals.  With  these 
minerals  are  always  associated  more  or  less  ferruginous  carbonate,  magnetite, 
hematite,  and  limonite.  In  these  rocks  we  find  developed  the  peculiar  oval,  crescent- 
shaped,  and  rounded  granules  which  are  so  characteristic  of  the  Biwabik  formation 
of  the  Mesabi  range — granules  made  up  in  their  freshest  condition  of  a  hjalrous 
ferrous  silicate  of  varying  shades  of  green.  These  rocks  have  been  extremely 
metamorphosed  by  the  Duluth  gabbro.  Where  most  metamorphosed  the  iron-bearing 
Gunflint  rocks  are  composed  of  coarsel}'  crystalline  bands  of  quartz,  of  varying 
width,  alternating  with  coarselv  crystalline  bands  of  magnetite  ore  reported  to  vary 
from  1  inch  up  to  10  or  12  feet  in  thickness,  and  of  bands  of  dark-green,  brown,  or 
black  rocks  that  consist  of  combinations  of  quartz,  augite,  hypersthene,  hornblende, 
olivine,  and  magnetite  as  the  principal  minerals,  but  associated  occasional^  with 


2G  OUTLINE  OF  MONOGRAPH. 

some  ferruginous  carbonate,  actinolite,  and  griinerite.  The  rounded  granules  are 
sometimes  preserved  in  tliese  rocks,  sliowing  their  derivation  from  the  least 
metamorphosed  forms  previouslj'  mentioned,  although  the  granules  consist  of 
minerals  different  from  those  in  the  least  metamorphosed  forms.  The  Rove  slate 
conformably  overlies  the  Gunflint  formation.  The  rocks  of  this  series  show  nothing 
of  especial  interest.  They  have  been  metamorphosed  slightly  as  a  result  of  the 
contact  action  of  the  adjacent  Duluth  gabbro  mass  and  the  intrusive  Logan  sills. 

Chapter  VI.  This  chapter  treats  of  the  Keweenawan  series.  The  only  rocks 
of  this  age  in  the  Vermilion  district  are  gabbros  forming  a  part  of  the  Duluth  gabbro 
mass  of  northeastern  Minnesota,  certain  great  basic  sills  to  which  the  name  Logan 
sills  has  been  given,  and  some  few  basic  and  acid  dikes  which  cut  all  the  rocks 
of  the  district,  including  the  aforementioned  Duluth  gabbro  and  the  Logan  sills. 
The  studies  of  the  writer  and  his  associates  have  been  confined  chiefly  to  the 
northern  edge  of  the  Duluth  gabbro,  which  appears  in  the  Vermilion  district. 
Several  reconnaissance  trips  have  also  been  made  into  the  area  underlain  by  the 
gabbro.  As  a  result  of  these  studies,  the  Duluth  gabbro  is  found  to  vary  in 
texture  from  a  coarse-grained  granular  rock  to  a  relatively  fine-grained  rock.  It 
also  has  in  places  a  gneissic  structure.  Under  the  microscope  the  texture  is  seen  to 
vary  from  granular  to  ophitic.  The  Logan  sills  are  great  masses  of  doleritic  rocks 
that  occur  for  the  most  part  as  sills  interbanded  with  the  Upper  Huroniau  sediments, 
and  at  times  cut  in  dike  form  across  them.  Petrographically  these  dolerites  range 
from  coarse-grained  rocks,  found  in  the  centers  of  the  sills,  with  an  imperfect 
granular  texture  and  verj^  similar  to  the  gabbro,  through  normal  ophitic  dolerites. 
to  intersertal  textured  basalts  on  the  selvages  of  the  sills.  The  gabbro  is  found  to 
metamorphose  all  of  the  sediments  already  enumerated,  and  is  thus  shown  to  be 
one  of  the  youngest  rocks  of  the  district.  It  is  also  found  to  be  intrusive  in  the 
Keweenawan  volcanics.  A  number  of  facts  are  enumerated  to  show  that  the  gabbro 
and  the  Logan  sills  are  of  essentialh"  the  same  petrographic  character,  although 
they  exhibit  minor  differences  that  are  readily  explicable  when  one  considers  the 
relative  amounts  of  the  two  rocks.  After  a  consideration  of  these  facts  and  of 
the  stratigraphic  relationship  of  the  rocks  the  conclusion  is  reached  that  the  gabbro 
and  the  sills  are  of  essentially  the  same  composition  and  age,  having  been  derived 
from  the  same  pai-ent  mass  of  magma.  In  certain  localities  in  the  Duluth  gabbro 
there  are  found  masses  of  titaniferous  magnetite  of  varying  size,  with  some  associated 
minei'als.  These  masses  grade  into  the  surrounding  gabbro,  and  were  formed  as  the 
result  of  processes  of  segregation.  No  published  description  has  yet  been  given,  so 
far  as  the  writer  knows,  of  any  large  continuous  masses  of  titaniferous  magnetite  in 
these  gabbros,  and  he  knows  of  none  from  personal  observation.  If,  however,  large 
masses  do  exist  their  content  of  titanium  would  prevent  them  from  being  of  value  at 
the  present  time,  when,  according  to  the  modern  iron-smelting  practice,  titaniferous 
ores  can  not  be  smelted  economically.  In  a  short  section  mention  is  made  of  the 
acid  dikes  that  are  younger  than  the  Duluth  gabbro,  and  of  certain  basalt  anddolerite 


OUTLINE  OF  MONOGRAPH.  27 

dikes  that  are  younger  than  the  gabbro  and  that  cut  the  acid  dikes,  which  themselves 
cut  the  gabbro. 

Chapter  VII.  In  this  chapter  the  drift  is  briefij^  described,  the  general  distribu- 
tion of  the  Vermilion  moraine  is  outlined,  and  the  locations  of  certain  glacial  lakes 
are  stated. 

Chapter  VIII.  This  chapter  consists  of  a  brief  discussion  of  the  topography  of  the 
district  in  its  relation  to  the  geologic  structure. 

Chapter  IX.  This  chapter  is  devoted  to  a  discussion  of  the  general  geologic 
history  of  the  district  as  determined  by  the  various  facts  set  forth  in  previous 
chapters. 


THE  VERMILION  IRON-BEARING  DISTRICT 

OF  MINNESOTA. 


By  J.  Morgan  Clements. 


CHAPTER  I. 

GENERAL  DESCRIPTION  OF  THE  DISTRICT. 

INTRODUCTIOIV. 

The  Vermilion  irou-beariug  district  of  Minnesota  is  like  all  of  the  other 
iron-bearing  districts  of  the  Lake  Superior  region  in  that  the  rocks  are  of 
verv  great  geologic  age.  Its  economic  importance  has  been  known,  how- 
ever, for  a  comparatively  short  period.  The  first  statement  of  the  existence 
of  iron  ore  in  this  district  is  credited  to  J.  Gr.  Norwood,  who  observed  it  upon 
his  explorations  in  1850  and  refers  to  it  in  his  report."  It  was  not  until  the 
early  eighties  that  a  determined  effort  was  made  to  develop  the  iron  resources 
which  some  then  knew  were  in  this  district.  In  1884  the  railroad  from 
Duluth  was  completed  to  Tower,  and  the  first  shipment  of  iron  ore  was  made. 
From  this  time  on  the  development  of  the  iron  resources  of  the  district  was 
rapid,  as  is  shown  by  the  annual  increase  in  the  shipments  of  ore.  This 
•increase,  with  minor  fluctuations  in  1893  and  1898,  caused  by  financial 
conditions,  continued  up  to  the  season  of  1902,  when  the  maximum  ship- 
ment for  the  district,  2,083,784  tons,  was  reached. 

PREVIOUS  GEOLOGIC  WORK  IN   THE  DISTRICT. 

Mr.  Bailey  Willis,  special  agent  of  the  Census  Office  of  the  United 
States,  spent  one  month,  October  10  to  November  10,  1880,  studying  the 

"Report  of  a  geological  survey  of  Wisconsin,  Iowa,  and  "Minnesota,  by  D.  D.  Owen,  1852,  report  of 

J.  G.  Norwood,  p.  417. 

29 


30  THE  VERMILION  IRON-BEARING  DISTRICT. 

geology  of  the  part  of  the  Vermilion  district  in  the  immediate  vicinity  of 
Tower.  In  1883-1885  Prof  R.  D.  Irving  spent  several  months  studying 
the  geology  of  this  district.  He  was  assisted  in  1883  by  Mr.  AY.  M. 
Chauvenet  and  in  1884  by  Mr.  W.  M.  Chau-\-enet  and  Mr.  W.  N.  ]\IeiTiam. 
These  studies  were  continued  in  1885  and  1886  by  Mr.  W.  N.  Meniam, 
assisted  in  1886  by  Mr.  W.  S.  Bay  ley  during  a  portion  of  the  season.  In 
1888  Prof.  C.  R.  Van  Hise  visited  the  district,  traversing  it  from  end  to  end. 
The  general  results  of  these  trijDS,  which  were  made  for  the  United  States 
Geological  Survey,  were  embodied  in  various  papers  which  are  refen-ed  to 
under  the  review  of  the  literature  (Chapter  II  of  this  monograph)  and  in 
manuscript  reports  that  are  preserved  in  the  office  of  the  Survey.  Various 
members  of  the  Minnesota  Geological  Surve}-  have  spent  parts  of  or  entire 
field  seasons  in  the  district,  and  their  results  are  published  in  the  reports  of 
the  State  survey. 

In  pursuance  of  a  plan  to  study  each  of  the  Lake  Superior  iron-bearing 
districts  and  make  detail  reports  on  them  the  United  States  Geological 
Survey  resumed  work  in  the  Vermilion  district  in  1897.  This  work  has 
been  under  the  general  charge  of  Prof.  C.  R.  Van  Hise.  The  geologists  in 
the  field  were  Messrs.  W.  S.  Bayley,  C.  K.  Leith,  and  J.  Morgan  Clements. 
The  field  work  continued  through  the  field  seasons  of  1897,  1898,  1899, 
and  1900.  Professor  Van  Hise  was  in  the  district  for  short  periods  daring 
the  diiferent  seasons,  and  in  1899  he  spent  a  large  part  of  the  season  in 
active  field  work.  Mr.  Bayley  spent  the  seasons  of  1897  and  1898  "in  the 
field;  Mr.  Leith  spent  the  seasons  of  1897, 1898,  and  1899;  and  Mr.  Clements 
(the  writer)  was  present  every  year,  remaining  throughout  the  entire  season. 

In  preparing  this  report  the  writer  has,  of  course,  made  use  of  the 
material  obtained  by  the  other  members  of  the  survey,  and  is  very  greatly 
indebted  to  them  for  the  assistance  given  by  their  carefully  prepared  notes. . 
He  is,  however,  chiefly  under  obligations  to  Professor  Van  Hise,  who,  in 
the  first  place,  gave  him  the  opportunit}-  to  prepare  the  report,  and  wlio 
has  ever  been  ready  to  assist  him  both  in  the  field  and  in  the  office. 

The  mining  men  of  the  Vermilion  district  have,  almost  without  exception, 
shown  high  appreciation  of  tlie  work  done  in  other  districts  by  the  United 
States  Geological  Survey,  and  have  rendered  all  legitimate  assistance 
witliin  their  power  during  the  progress  of  the  work.  The  Minnesota  Iron 
Coinpanv,  under  the  presidencj-  of  'Sh:  D.  H.  Bacon,  and  later  of  ]\Ir.  T.  F. 


INTRODUCTION.  31 

Cole,  gHYe  invaluable  aid.  In  1899  the  company  began  a  careful  geologic 
survey  of  its  lands,  which  was  made  in  far  greater  detail  than  was 
possible  by  the  United  States  Geological  Survey  under  existing  conditions. 
This  private  survey  was  carried  out  by  Mr.  W.  N.  Merriam,  assisted  in 
1900  by  Mr.  Oscar  Rohn  All  of  the  material  resulting  from  this  survey 
has  been  placed  at  the  disposal  of  the  writer  and  his  collaborators;  a  great 
deal  has  been  used  in  compiling  the  maps  published  herewith,  and  it  has 
added  very  materially  to  their  comj^leteness.  Moreover,  Mr.  Merriam  has 
taken  pains  to  make  drawings,  some  of  which  are  reproduced  in  this  report 
(credited  to  him),  and  otherwise  to  render  assistance.  To  the  company 
which  he  represents,  and  to  him  especially,  the  United  States  geologists  are 
deeply  indebted.  The  writer  wishes  also  to  acknowledge  here  the  great 
assistance  rendered  by  Mr.  E.  R.  Maurer  and  Mr.  C.  F.  Graff,  who  have 
prepared  the  drawings  from  which  the  maps  and  plates  are  made,  and  by 
Mr.  F.  B.  Van  Horn,  his  efficient  stenographer. 

SCOPE   OF   THE   PAPER. 

The  attempt  has  been  made  to  make  this  repovt  a  complete  epitome  of 
our  knowledge  of  the  Vermilion  district.  At  the  same  time  many  details 
have  necessarily  been  omitied,  although  in  most  cases  these  concern  the 
formations  of  the  district  that  are  not  of  economic  value  and  are  not  likely 
to  become  important.  These  details,  without  adding  to  the  g'eheral  results, 
would  have  very  much  increased  the  bulk  of  the  volume.  Moreover,  it 
was  feared  that  they  would  obscure  important  facts  and  thus  defeat  the 
object  of  the  monograph. 

The  report  is  intended  jjrimarily  to  give  to  mining-  men  and  to  jjresent 
and  prospective  owners  of  property  in  the  district  information  concerning 
the  distribution  of  the  important  iron-bearing  formations  and  their  relations 
to  the  other  rocks  associated  with  them.  The  text  gives  a  full  description 
of  these  formations. 

The  atlas  of  maps  and  the  plates  in  the  volume  are  for  the  purpose  of 
aiding  in  an  understanding  of  the  textual  descriptions.  Actually  observed 
exposures  of  the  rocks  of  the  district  could  not  be  indicated  in  all  cases  on 
the  maps  because  their  scale  is  too  small.  Large-scaled  maps  of  certain 
portions  of  the  district  that  contain  the  important  iron-bearing  formation 
in  its  best  development,  and  in  which  areas  any  industrial  developments 


32  THE  VERMILION  IRON-BEARING  DISTRICT. 

of  tlie  future  are  likely  to  occur,  have  been  prepared.  On  these  plates 
the  exposures  actually  seen  during  the  field  AA-ork  have  been  indicated, 
drawn  to  very  nearly  correct  scale.  Man}-  of  the  exposures  are  so  small 
that  in  order  to  represent  them  on  the  maps  at  all  It  has  been  necessary  to 
exaggerate  them.  On  these  maps  are  given  the  data  which  were  tised  as 
the  basis  for  drawing  the  formation  lines  in  these  economically  important 
areas.  If  anyone  is  unable  to  accept  the  conclusions  reached,  he  may 
draw  his  own  inferences  from  the  data  given,  which  are  essentially  connect, 
although,  as  will  be  understood  by  those  who  know  the  limitations  under 
which  such  geologic  mapping  is  done,  a  number  of  minor  errors  may  be 
disclosed  by  very  close  work  and  very  exact  location  of  exposures  with 
instruments  of  precision.  All  geologic  maps  are  but  approximations  to  the 
truth.  The  aim  has  been  to  make  the  present  apjDroximation  as  close  as 
practicable. 

A  great  deal  of  time  has  been  devoted  to  examining  all  available  literature 
that  refers  in  any  way  to  this  district.  Fairly  complete  quotations  are  made 
from  the  various  works  cited,  so  that  a  careful  reading  of  the  review  of  the 
literature  will  enable  one  to  familiarize  himself  with  the  changes  of  opinions 
concerning  the  structure,  character  of  rocks,  and  other  details,  and  also  with 
the  gradual  increase  in  knowledge  concerning  the  geology  of  the  district. 

GEOGRAPHIC   LIMITS. 

The  territory  designated  by  the  name  "  Vermihou  iron-bearing  district" 
lies  in  the  extreme  northeastern  portion  of  Minnesota,  including  portions 
of  St.  Louis,  Lake,  and  Cook  counties.  The  district  has  an  area  of 
approximately  1,000  square  miles.  It  is  a  narrow  belt  extending  east- 
northeast  from  near  the  west  end  of  Vermilion  Lake,  in  longitude  92°  30' 
west  from  Greenwich,  on  the  west,  to  the  vicinity  of  Gunflint  Lake,  on  the 
international  boundary,  on  the  east,  in  about  longitude  90'^  45'  west  from 
Greenwich.  The  district  lies  between  47°  15'  and  48°  15'  north  latitude. 
It  attains  its  maximum  width  at  the  west,  where  it  is  about  18  miles  wide, 
and  gradually  narrows  eastward,  until  at  Gunflint  Lake  its  minimum  width 
is  2  miles.  The  geographic  relations  of  the  Vermilion  iron-bearing  district 
of  Minnesota  to  the  other  iron-bearing  districts  of  the  Lake  Superior  region 
can  be  seen  on  PI.  I. 

The  western  limit  of  the  district  as  given  on  the  map  (92°  30')  is  purely 
arbitrary.    West  of  this  the  country  is  heavily  drift  covered  and  timbered,  and 


U,  S-  GEOLOGICAL   SURVEY 


MONOGRAPH    XLV   PL    I 


.VRCHK.^.N-  ;^^^i^-''-^^'L 


PO  ST^fVLGON  KIAN 
nUROXIAN  EEWEEKAWAK 

\_/R__]  HHl  [I^KU  I     Pa     I 

Including  considerable      lncladui|),  diiisidernhle 
areas  orAlgunkuui  ^raiute      areas  of  Arch  e  mi 

I  llie  iron -hearing  3ai<'s ' 

GEOLO(ViC  MAP  OF  VMi'V  OF  THH   LAIvJ^  SUl^KRIOli  ItKUlOJs 

SlTOUTJ«IO   PRK-rAMHlUAN'  HOCKS 

('aiiipilednMjuoffi[-iaJ  jimpsur  ITnitcd  SlAtes.SMle.aiidC&iiaUuui  survfyf. 

SchJc 


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i\ada 

STRATIGRAPHY.  33 

great  miiskegs"  extend  over  large  areas.  In  that  region  outcrops  are  very 
scarce,  but  exposures  of  banded  jasper  and  iron  ore  have  been  found  associ- 
ated with  greenstones,  granite,  and  clastic  sediments.  A  reconnaissance 
trip  was  made  through  it  in  1886  by  Mr.  W.  N.  Merriam  for  the  United  States 
Geological  Survey,  and  this  work,  as  well  as  that  done  in  that  portion  of  the 
district  for  private  corporations,  to  whose  results  the  Survey  has  had  access, 
shows  the  futility  of  attempting  at  present  to  trace  out  formation  lines  in  that 
region.     Hence  the  Survey  has  done  no  detailed  work  west  of  the  above  line. 

The  same  rocks  that  occur  at  the  eastern  end  of  the  district  are  known  to 
continue  for  many  miles  eastward,  both  in  the  United  States  and  in  Canada, 
Hence  this  eastern  limit  also  is  arbitrary,  and  includes,  indeed,  rocks  that  are 
the  direct  eastward  continuation  of  the  topographic  feature  known  as  the  Me- 
sabi  range,  which  in  the  western  part  of  the  district  lies  south  of  the  Vermilion. 

The  southern  and  northern  limits  are  sharply  defined,  and  are  well 
marked  geologically  by  granite  and  gabbro.  The  gabbro  bounds  only  the 
southern  side  of  the  district  in  the  eastern  part. 

STRATIGRAPHY. 

The  stratigraphic  succession  in  the  Vermilion  district  is  as  follows,  in 
descending  order : 

Pleistocene ,  - .  Drift. 

Ke weenawan Duluth  gabbro  and  Logan  sills. 

( Unconformity. ) 
Upper  Huroiiian  (Animikie  series).    ConijRove  slate. 

fined  to  east  end  of  district  iGunflint  formation  ( iron-bearing) . 


(Unconformity. 

Lower  Huronian 

( Unconformity. 

Archean i ._ 


Intrasives.     Granites,    granite-porphyries,    dolerites,   and 

lamprophyres. 
Knife  Lake  slates. 
Agawa  formation  ( iron-bearing ) . 
Ogisbke  conglomerate. 

Intrusive  gi'anites,    granite-porphyries,   and   some  green- 
stones. 

Soudan  formation  (the  iron-bearing  formation). 
(Minor  unconformity. ) 

Ely  greenstone,  an  ellipsoidally  parted  basic  igneous  and 
largely  volcanic  rock. 


«  These  muskegs,  as  they  are  called  by  the  Indians,  are  great  open  swamps  that  are  comparable 
in  a  way  with  the  northern  tundras.  They  have  been  formed  in  most  cases  by  the  drying  up  of  large 
bodies  of  water,  and  in  many  of  them  there  is  now  an  open  area  occupied  by  the  remnant  of  a  larger 
lake.  Over  the  area  surrounding  the  water  there  is  spread  a  growth  made  up  largely  of  sphagnum 
moss,  wild  cranberry  bushes,  and  other  water-loving  plants,  with  occasional  swamp-growth  shrubs  that 
attain  a  height  of  1  to  3  feet.  Out  of  this  thick  undergrowth  there  rise  isolated  tamarack  and  spruce 
trees,  usually  of  small  size.  Where  these  muskeg  swamps  border  the  large  lakes  they  are  sometimes 
flooded  during  high  water. 

MON  XLV — 03 3 


34  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  evidence  upon  which  the  above  formations  are  grouped  into  series 
and  these  correlated  witli  the  formations  in  other  districts  of  the  Lake 
■Superior  region  will  appear  in  subsequent  pages.  In  this  place  is  given 
merely  a  categorical  statement  of  the  problems  to  be  treated. 

The  accompanying  general  map  (PI.  11)  shows  the  distribution  of  the 
various  formations  enumerated  above.  The  reader  is  able  to  get  a  better 
idea  of  the  relationship  of  the  formations  and  their  distribution  throughout 
the  entire  district  from  a  study  of  this  general  map  than  he  could  from  the 
examination  of  the  larger-scale  maps  in  the  atlas,  which  are  of  relatively 
small  areas.  On  the  larger-scale  sheets,  in  the  accompanying  atlas,  details 
of  topography  are  shown  which  could  not  be  shown  on  the  general  map. 
These  atlas  sheets  and  the  other  more  detailed  sheets  on  a  still  larger 
scale  are  more  accurate  than  the  general  map  and  should  be  used  in  a 
detailed  geologic  study  of  the  district  having  in  view  the  location  of 
possible  productive  properties. 

PHYSIOGRAPHY. 

RELIEF. 

That  portion  of  Minnesota  included  within  the  limits  given  above  is 
most  commonly  known  in  commercial  reports  and  locally  as  the  "Vermil- 
ion iron  range."  The  term  "range"  is  in  this  case,  however,  a  misnomer, 
if  one  understands  thereby  an  area  with  strongly  marked  topographic 
features  which  cause  it  to  stand  out  from  the  adjacent  areas.  The  Ver- 
milion district  is  one  in  which  the  relief  is  not  very  great.  The  maximum 
elevation  is  attained  by  a  hill  in  sec.  28,  T.  65  N.,  R.  4  W.,  near  the  east 
end  of  the  district,  which  reaches  a  height  of  2,120  feet  above  sea  level,  or 
1,518  feet  above  the  mean  level  of  Lake  Superior,  which  is  601.5(3  feet 
above  the  sea.  This  is  one  of  the  highest  points  in  the  State,  the  highest 
hill  having  a  reported  altitude  of  2,230  feet."  The  lowest  valley  is  that 
occupied  by  Bass  wood  Lake,  in  which  the  water  level  is  1,300  feet  above 
the  sea,  or  698  feet  above  Lake  Superior.  There  is,  then,  a  difference  of  820 
feet  between  the  lowest  water  level  and  the  highest  hill  within  tlie  district. 
The  above  extremes  in  height  are  found  at  opposite  ends  of  the  district, 
and,  as  the  general  slope  is  to  the  northwest,  the  average  relief  is  very 
much  less  than  820  feet,  approaching  400  or  500  feet.     It  is  to  be  further 


"Geol.  ami  Xat.  Hist.  Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1S99,  p.  481. 


I    S    GEOLOGrCAL  SURVF 


RELIEF.  35 

noted  that  in  general  the  changes  in  rehef  are  very  rapid,  and  as  a  conse- 
quence the  district  is  extremely  rugged  in  detail,  diversified  by  hills  and 
many  lakes,  with  an  occasional  muskeg.  Over  the  greater  portion  of  the 
area  we  find  east-northeast  trending  ridges  alternating  with  valleys  occupied 
by  long  lakes,  or  chains  of  lakes,  or  streams.  As  a  consequence,  in 
traversing  the  district  from  north  to  south  one  is  continually  ascending  a 
steep  ridge  to  descend  on  the  opposite  side  into  a  valley  which  is  usually 
occupied  by  a  lake. 

The  Vermilion  district,  considered  broadly,  may  be  divided  into  four 
areas,  each  of  which  is  characterized  by  a  fairly  distinct  kind  of  topographic 
development.     These  are: 

(1)  The  area  including  the  Giants  range,  which  is  the  most  prominent 
topographic  feature  of  the  Vermilion  district. 

(2)  A  broad  area  north  and  northwest  of  the  Giants  range,  including 
all  the  areas  underlain  by  the  iron-bearing  formation.  This  is  very  rugged, 
but  the  differences  in  altitude  are  not  great. 

(3)  x\n  area  of  high  plateau  country  southeast  of  the  Giants  range, 
underlain  by  gabbro. 

(4)  •  A  small  triangular  area  at  the  extreme  eastern  end  of  the  district. 
The  apex  of  the  triangle  is  toward  the  west,  and  lies  between  the  Giants 
range  on  the  north  and  the  high  plateau  to  the  south. 

Mesabi  or  Giants  range. — This  is  a  fairly  well-marked  east-northeast 
trending  range  of  hills,"  which  runs  obliquely  across  the  district.  It  forms 
the  backbone  of  the  Vermilion  district,  although  it  is  unsymmetrical  and 
divides  the  district  into  unequal  areas.  It  enters  the  district  in  T.  62  N., 
R.  12  W.,  and  extends  in  an  east-northeast  direction  along  the  Kawishiwi 
River,  south  of  Snowbank  Lake  and  Cacaquabic  Lake,  and  north  of 
Lake  Gobbemichigamma  to  the  east  side  of  T.  65  N.,  R.  4  W.,  where  it 
leaves  the  district  and  enters  Canadian  territory.^  The  maximum  height 
of  this  range  is  attained  by  a  liill  in  sec.  28,  T.  65  N.,  R.  4  W.,  already 

« This  range  has  been  known  as  the  Mesabi  for  an  unknown  length  of  time  by  the  Indians 
inhabiting  this  region,  and  has  been  so  called  in  the  reports  of  Western  explorers  or  else  translated  by 
them  into  Giants  range.  In  late  reports  Prof.  N.  H.  Winchell  has  applied  the  term  Mesabi  to  a  range 
of  hills  lying  south  of  that  known  as  the  Giants  range  proper,  to  which  the  above  statements  apply. 
Winchell  has  his  ilesabi  and  Giants  range  proper  unite  a  short  distance  southwest  of  Birch  Lake  and 
form  the  Giants  range  to  the  east.  West  of  this  point  he  discriminates  the  range  into  the  Giants  range 
to  the  north  and  the  ^lesabi  to  the  south.  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Thirteenth 
•  Ann.  Kept.,  1885,  p.  22;  Final  Kept.,  Vol.  IV,  1899,  p.  232. 

f'N.  H.  AVinchell,  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Thirteenth  Ann.  Rept.,  188-5,  p.  38. 


36  THE  VERMILION  IRON-BEARING  DISTRICT. 

referred  to  as  the  liig-liest  hill  in  the  district,  which  reaches  2,120  feet 
above  sea  level.  This  is  about  460  feet  above  the  general  level  of  the 
surrounding-  country.  As  a  general  thing  the  range  does  not  stand  out 
very  prominently  from  the  rest  of  the  district.  Between  Gobbemichi- 
gamnia  and  Cacaquabic  lakes,  however,  there  is  a  subordinate  range,  with 
Twin  Peaks  as  the  highest  points,  which  forms  a  very  prominent  feature  of 
this  part  of  the  district.  The  Giants  range  is  not  continuous  throughout. 
It  is  made  up  of  a  great  number  of  small  hill  ranges  having  in  general 
the  trend  of  the  main  range  to  which  the}"  belong.  Its  contours  are 
commonly  smooth  and  rounded,  as  the  result  of  glaciation.  On  its  slopes 
are  inany  minor  irregularities  caused  by  glacial  deposits.  Among  these 
deposits  we  find  now  small  glacial  lakes  almost  upon  the  summit 
of  the  range. 

Area  north  and  nortluvest  of  Giants  range. — It  has  been  said  that  the 
Giants  range  divides  the  Vermilion  district  topographically.  The  area 
with  the  largfest  surface  extent  is  that  lying  north  and  northwest  of  the 
rauffe.  This  area  merg-es  to  the  south  into  the  Giants  range,  and  continues 
to  the  north  beyond  the  limits  of  the  area  mapped.  Within  this  area  the 
topography  is  that  which  has  been  brieflj'  described  on  p.  35  as  fairly 
typical  for  the  entire  district.  It  consists  in  ridges  trending  N.  60°-80°  E., 
and  separated  by  valleys  which  are  usually  occupied  liy  a  long  lake,  a 
string  of  small  lakes,  or  a  stream.  The  ridges  are  usually  about  200  feet 
above  the  lakes.  The  greatest  height  in  this  portion  of  the  district  is 
reached  by  Chester"  or  Jasper  Peak,  in  sec.  35,  T.  62  N.,  R.  15  W.,  which 
is  1,710  feet  above  sea  level.  The  topography  is  less  rugged  in  the 
western  part  of  the  district,  where  the  hills  and  ridges  have  been  apparently 
more  affected  by  glaciation.  They  are  there  generally  rounded  and  the 
slopes  are  much  gentler  than  in  the  eastern  portion.  In  the  east  the  area 
is  underlain  by  a  great  slate  formation,  and  the  jointing  of  the  slates  has 
caused  the  development  of  minor  drainage  lines  and  ridges  transverse  to 

"The  name  Chester  was  the  first  recorded  name  given  to  this  pealv  by  \vliite  men.  It  was  so 
called  in  honor  of  Prof.  A.  H.  Chester,  who  did  the  first  imjiortant  work  towawl  exploiting  the  iron 
deposits  of  this  district.  The  peak  is  the  most  i)roniinent  topographic  feature  of  this  part  of  the 
district.  It  is  an  alnfost  bare  knob  of  ja.sper.  This  jasper  is  one  of  the  important  rocks  of  the 
Lake  Superior  iron  region,  and  as  everyone  is  more  or  less  familiar  with  it,  the  peak  has  naturally 
been  called  after  the  rock  of  which  it  is  formed.  The  writer  thinks  that  it  will  be  impossible  to  cause 
the  name  Chester  to  be  generally  used,  although  by  pricirity  this  name  rightfully  should  be  given  to 
the  peak. 


RELIEF.  37 

the  trend  of  the  other  topographic  features.  Moreover,  the  slates  break 
off,  forming  steep  chffs  that  stirround  the  ridges  and  hills,  instead  of  the 
moderate  slopes  more  common  in  the  western  part  of  the  district.  The 
valleys  are  almost  flat  and  are  exceptionally  wide.  In  short,  the  wide 
U-shaped  form  is  the  common  one  here,  rather  than  the  flaring  V-shaped 
valley  characteristic  of  rivers.  The  modification  of  the  topography  from 
the  V-shaped  to  the  U-shaped  forms  is  attributed  to  glacial  erosion  and 
deposition. 

One  area  in  which  rather  interesting  topography  was  observed  is 
that  extending  from  about  1^-  miles  southeast  of  Ely,  in  sec.  2,  T.  62  N., 
R.  12  W.,  southwestward  to  sec.  30,  T.  62  N.,  R.  12  W.,  including  about 
14  square  miles.  This  area  is  underlain  by  the  Griants  Range  granite,  and 
throughout  the  relief  is  very  slight,  the  greater  portion  of  the  land  surface 
being  only  a  foot  or  so  above  the  level  of  the  lakes.  As  a  result  the  major 
portion  of  it  is  a  marsh.  The  knolls  are  of  granite,  with  low,  rounded 
surfaces  rising  only  a  few  feet  above  the  swamp  area.  A  few  of  the  knolls 
are  composed  of  glacial  drift.  Evidently  pre-Grlacial  drainage  had  been 
especially  vigorous  here,  and  this  is  a  small,  nearlv  base-leveled  area,  with 
the  lakes  as  the  base-level. 

On  a  reconnaissance  trip  along  the  international  border  a  similar 
area  was  noted  surrounding  the  southeast  side  of  Iron  Lake,  in  sees. 
11  and  12,  T.  66  N.,  R.  13  W.,  and  sec.  7,  T.  66  N.,  R.  12  W.,  outside  of 
the  Vermilion  district.  Here  base-leveling  has  proceeded  farther  than  in 
the  area  previously  mentioned;  the  islands  in  this  portion  of  the  lake  rise 
just  above  the  water  level,  and  the  small  streams  entering  the  lake  here 
flow  with  meandering  courses  through  wide  marshes. 

The  gabbro  plateau. — In  northeastern  Minnesota,  southeast  of  the 
Vermilion  district  pi'oper,  there  is  a  large  area  underlain  by  gabbro.  Only 
a  small  portion  of  this  area  comes  within  the  region  shown  on  the  accom- 
panying general  map,  and  that  portion  is  a  strip  on  its  southern  and 
southeastern  edge.  Knowledge  of  the  plateau  has  been  derived  from  a 
study  of  this  strip,  where  for  the  most  part  stratigraphic  work  ended;  from 
the  results  of  a  reconnaissance  trip  within  the  area  underlain  by  the 
gabbro;  and  chiefly  from  the  description  by  Dr.  U.  S.  Grant  in  the  last 
Minnesota  report, "  to  which  the  reader  is  referred  for  greater  detail.     Dr. 

"Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  pp.  434-436. 


38  THE  VERMILION  IRON-BEARING  DISTRICT. 

Grant  describes  tlie  gabbro  area  as  an  elevated  plateau,  upon  whose  surface, 
however,  there  are  many  minor  irregularities,  but  few  of  which  rise  100 
feet  above  the  surrounding  country.  Lakes  are  common,  but  very  shallow. 
"The  general  plain-like  character  of  the  gabbro-covered  area  can  be 
ascribed  to  weathering,  erosion,  and  glaciation  acting  on  a  surface  composed 
of  a  single  rock  mass  (the  gabbro)  uniform  in  constitution,  grain,  and 
resistance  to  disintegrating  agents."" 

Gimflmt  Lal-e  area. — Near  the  east  end  of  the  district,  as  shown  on 
the  map  (PI.  II),  there  is  an  area  having  approximately  the  shape  of  an 
isosceles  triangle,  with  the  apex  of  the  triangle  pointing  west.  The  north 
side  is  bounded  by  the  Griants  range,  the  south  side  by  the  gabbro 
plateau,  and  the  base  is  the  eastern  limit  of  the  region  sho\\'n  on  the 
general  map.  Within  this  area  a  very  interesting  kind  of  topography  is 
developed.  Tlie  following  description  is  based  upon  a  personal  visit  to 
the  greater  portion  of  the  area  described,  and  upon  the  published  reports  of 
Dr.  U.  S.  Grant." 

This  area  is  underlain  by  a  series  of  Upper  Huronian  slates  which 
have  a  low  dip  to  the  south-southeast.  The  continuity  of  the  slate  series 
is  interrupted  by  numerous  sills  of  coarse  dolerite  of  varying  thickness, 
which  were  intruded  approximately  parallel  to  the  beds.  Pre-Glacial 
erosion  developed  here  a  system  of  east-west  ridges  and  valleys.  Each 
rido-e  is  capped  by  a  layer  of  dolerite  and  below,  protected  by  this  hard 
upper  layer,  lie  the  slates.  The  ridges  slope  gently  to  the  south.  The 
slope  follows  approximately  the  upper  side  of  the  dolerite  sill  and  corre- 
sponds closely  to  the  dip  of  the  sediments.  To  the  north  the  ridges  break 
off  abruptly,  giving  a  steep  or  precipitous  escarpment  with  a  talus  below. 
The  narrow  valleys  between  the  ridges  are  usually  occupied  by  lakes,  and 
each  lake  is  higher  than  the  one  in  the  valley  next  north  of  it.  A  cross 
section  would  show  on  the  north  a  talus  surmounted  by  a  clitf  which  forms 
the  brow  of  the  ridge,  the  latter  sloping  gentl)-  to  the  south  to  the  valley, 
which  contains  a  lake.  Then  the  talus,  cliff,  and  slope  are  repeated.  Such 
a  north-south  section  would  have  ranch  the  jagged  appearance  of  the  edge 
of  a  saw.  From  this  character  these  hills  are  frequently  spoken  of  as  the 
"sawtooth  hills." 


"(Trant,  u]).  cit.,  p.  4o5.  ''(.>ii.  i-il.,  p|i.  •'!17.  4S2. 


RELIEF  AND  DRAINAGE.  39 

The  entire  district  has  been  overrun  by  glaciers,  and  in  consequence 
glacial  drift  is  widely  distributed.  In  places  it  is  very  thin,  liut  in  other 
places  it  has  accumulated  to  a  considerable  depth  over  large  areas.  In 
these  areas  the  striking  features  of  the  topogi-aphy  are  due  essentially 
to  the  drift.  However,  the  present  relief  can  easily  be  seen  to  have  been 
superimposed  upon  a  pre-Glacial  topog'raphy  of  very  different  character. 
A  deep  covering  of  drift  occurs  in  all  four  of  the  greater  areas  above 
outlined,  and  modifies  the  topography  locally. 

DRAINAGE. 

One  can  not  glance  at  the  topographic  maps  of  the  Vermilion  district 
in  the  accompanying  atlas  without  being  impressed  by  the  abundance  of 
lakes  in  it.  These  numerous  lakes,  with  their  connecting  streams,  make 
the  district  comparatively  easy  of  access.  They  have  enabled  us  to  study 
the  geology  with  a  much  smaller  expenditure  of  time  and  money  than 
would  be  requii-ed  if  they  did  not  exist.  The  presence  of  these  lakes 
is  clearly  indicative  of  an  immature  drainage  S3'stem,  which  is  further 
shown  by  the  absence  of  streams  of  large  size,  by  the  fact  that  the 
small  and  short  streams  which  do  exist  merely  serve  to  connect  the 
lakes  into  strings,  and,  by  the  fact  that  these  streams  are  frequently 
interrupted  in  their  courses  by  rapids  and  falls.  The  presence  of  large 
muskegs  in  some  portions  of  the  district  still  further  emphasizes  this  very 
imperfect  drainage. 

Sydrograpliic  basins. — These  lakes  and  the  streams  which  feed  and  drain 
them  belong  to  the  large  hydrographic  basins  of  the  St.  Lawrence  River 
and  Hudson  Bay.  The  area  belonging  to  the  St.  Lawrence  drainage  basin 
is  very  small.  It  is  drained  by  a  small  stream — the  headwaters  of  the 
Embarrass  River,  a  tributary  of  the  St.  Louis — which  rises  in  Putman 
Lake,  sec.  18,  T.  61  N.,  R.  14  W.,  and  flows  south,  finally  emptying  into 
Lake  Superior.  It  is  interesting-  to  note  that  this  small  stream,  flowing 
south,  runs  for  a  considerable  distance,  in  the  area  outside  of  the  district 
mapped,  nearly  parallel  with  Pike  River,  a  stream  5  miles  west  of  it,  whose 
waters  flow  north  and  belong  to  the  Hudson  Bay  drainage. 

By  far  the  greater  part  of  the  district  belongs  to  the  drainage  basin  of 
Hudson  Bay.  The  waters  in  this  district  flow  north  and  west,  collecting- 
fin  ally  in  Rainy  Lake  and  draining-  through  the  river  of  the  same  name  into 


40  THE  VEKMILION  IRON-BEARING  DISTRICT. 

the  Lake  of  the  Woods  and  Lake  Wmnepeg,  and  finally  entering  Hudson 
Bay  through  the  Nelson  Eiver.  It  was  in  the  country  reached  by  means 
of  this  string  of  connecting  waters  that  the  great  battle  of  commercial 
supremacy  in  the  Northwest  was  fought  in  the  early  part  of  the  century  by 
the  Hudson  Bay  Company  and  its  younger  rival,  the  Northwest  Fur  Com- 
pany. The  Hudson  Bay  people  came  up  the  Nelson  and  carried  out  their 
goods  for  the  most  part  the  same  way.  The  Northwest  Fur  Company  came 
from  Lake  Superior  and  went  to  a  great  extent  through  the  streams  and 
lakes  bordering  the  Vermilion  district  down  the  Rainy  River  and  returned 
over  the  same  route,  although  at  times  an  all-Canadian  route  north  of  the 
international  boundary  was  used. 

Streams. — The  streams  of  the  district  are,  with  one  exception,  short, 
narrow,  and  shallow,  and  form  merely  the  connections  between  the 
numerous  lakes.  The  Kawishiwi"  River,  which  runs  through  T  63  N., 
Rs.  9,  10,  and  11  W.,  is  the  exception.  This  is  a  fairly  long  stream,  which 
for  a  portion  of  its  course  is  within  the  southern  border  of  the  district. 
In  places  it  is  both  wide  and  deep.  It  is  interrupted,  however,  by  rapids, 
and  is  full  of  widenings  which  really  may  be  considered  as  lakes,  so  that 
by  a  strict  interpretation  it  could  perhaps  be  classed  with  the  other  strings 
of  lakes.  The  course  of  this  stream  can  be  followed  on  the  accompanying 
maps,  and  an  examination  of  the  geologic  map  shows  that  it  follows  the 
contacts  of  the  various  formations  occurring  in  the  part  of  the  district  in 
which  it  runs.  The  Kawishiwi  River  is  peculiar  in  certain  portions  of 
its  course  and  shows  clearly  that  over  a  greater  portion  of  its  extent  it  is, 
as  has  already  been  stated,  merely  a  string  of  lakes.  The  main  stream 
flows  through  sees.  20  and  21,  T.  63  N.,  R.  9  W.,  just  below  the  margin  of 
the  area  mapped.  At  this  place  there  is  a  large  island,  about  2  miles  long, 
extending  northeast-southwest,  but  not  caused,  as  one  would  naturally 
think,  by  the  stream  di\dding  and  flowing  around  both  sides  of  it.  It  is 
due  to  the  fact  that  to  the  north  of  the  island,  in  sec.  16,  T.  63  N.,  R.  9  W., 
there  is  a  lake  which  has  two  outlets,  and  from  which  the  water  flows 
to  the  southwest  and  to  the  northeast.  The  Avater  running  southwest 
joins  the  Kawishiwi  after  flowing  about  one-fourth  of  a  mile.     The  water 

"The  Indian  name  of  this  river  is  reported  to  l)e  Mishiwishiwi,  meaning  "Big  Beaver 
House  River."  The  Minnesota  maps  give  it  as  Kawishiwi,  and  it  is  l:nown  in  local  parlance  as  the 
Cashawav. 


DRAINAGE.  ^  41 

flowing  northeast  follows  this  course  for  a  short  distance,  then  turns  to  the 
southwest  and  joins  the  Kawishiwi  about  2  miles  from  the  outlet  of  the  lake. 
Somewhat  farther  west  there  is  a  striking  case  of  the  division  of  the 
streain  and  the  formation  of  an  island  in  this  way.  The  island  here  referred 
-to  is  only  partly  included  in  the  area  shown  on  the  present  map,  and 
the  reader  is  referred  to  plate  78,  Vol.  IV,  Geological  and  Natural  History 
Survey  of  Minnesota,  Final  Report,  where  the  course  of  the  river 
around  it  is  shown.  The  area  is  briefly  described  by  Grant,  page  400  of 
the  same  report.  Extending  through  T.  62  N.,  Rs.  10,  11,  and  12  W.,  is 
a  laro'e  island,  with  a  maximum  lensfth  of  about  1 1  miles  and  a  breadth 
of  4  miles,  which  is  completely  surrounded  by  the  north  and  south 
branches  of  the  Kawishiwi  and  the  lakes  which  are  developed  in  their 
courses.  The  water  of  the  main  Kawishiwi  divides  in  sec.  26,  T.  63  N., 
R.  10  W.  A  part  of  it,  forming  the  north  branch  proper,  flows  nearly 
due  west  a  distance  of  about  8  miles,  in  which  distance  it  descends 
about  70  feet.  That  portion  forming  the  south  branch  flows  south  and 
southwest,  then  north  and  northeast.  The  total  length  of  the  Soutii  Kawish- 
iwi is  about  30  miles,  and  it  d.escends  70  feet  before  it  joins  the  first  or  the 
nortli  branch. 

Lakes. — The  lakes  are  the  most  characteristic  drainage  feature  of 
the  Vermilion  district.  They  are  a  source  of  great  relief  to  the  geologist, 
who,  wearied  with  a  day's  trainp  through  the  brush  on  the  hills,  returns 
to  his  birch-bark  canoe  and  paddles  back  to  his  camp,  situated  at  some 
pleasant  spot  on  the  shore.  They  likewise  aiford  a  constant  source  of 
pleasure  to  the  traveler  through  the  district,  whose  interest  is  ai'oused. 
by  the  rapid  changes  from  a  narrow  lake  with  rocky  clifi"s  to  others 
showing  broad  reaches  of  open  water  studded  with  green  islets.  This 
interest  is  sustained  by  the  fact  that  each  succeeding  lake  entered  affords 
something  new  to  attract  the  attention.  The  scenery,  although  all  of  the 
same  general  character,  is  constantly  changing  in  its  details.  Occasionally 
a  moose  or  caribou  may  be  seen  swimming  from  shore  to  shore,  and  the 
fishing-  is  generally  excellent.  The  lakes  vary  greatly  in  size.  Vermilion 
Lake,  the  largest,  covers  about  70  square  miles,  excluding  islands,  more 
than  fifty  of  which  are  in  the  area  mapped.  Other  large  lakes  are  Bass- 
wood  (Bassimenan  or  Whitewood)  Lake  and  Saganaga  Lake,  which  border 
the  district.     From  these  large  ones  the  lakes  grade  in  size  down  to  mere 


42  THE  VERMILION  IRON-BEARING  DISTRICT. 

ponds.     A  rough  coimt  gives  250  lakes  within  the  region  shown  on  the 
topographic  map. 

The  hikes  are  somewhat  scattered  in  the  western  portion  of  the  district, 
but  are  far  more  numerous  in  the  eastern  part.  There  are  many  small 
ones  with  no  visible  surface  outlet,  and  these  are  usually  completely 
surrounded  bv  drift.  The  lakes  lie  in  basins  that  trend  northeast-south- 
west, and  with  their  short  connecting  streams  they  constitute  the  main 
routes  of  travel  through  the  district.  Indeed,  were  it  not  for  them  the 
district  could  be  traversed  only  with  extreme  difficulty.  A  trip  north 
and  south  across  the  district  is  very  arduous,  as  the  trails,  if  any  exist,  are 
lono-  and  over  high,  rough  ridges,  while  the  waterways  are  so  naiTOW  in  this 
direction  as  to  make  it  seem  useless  to  carry  canoes  for  such  long  distances, 
in  \aew  of  the  short  distance  they  can  be  used,  whereas  without  them  such 
a  route  would  be  thoroughly  impracticable.  On  an  east-west  trip,  however, 
one  can  start  from  Tower,  and  go  along  the  lines  of  lakes  to  CTunflint,  on 
the  Canadian  border,  by  canoes — a  distance  of  75  miles  in  a  straight  line, 
and  much  greater  hj  the  route  traveled — and  on  the  trip  make  only  about 
20  portages,  not  aggregating  in  all  over  4  miles.  A  number  of  these 
portages,  moreover,  are  mere  lift  overs,  from  10  to  50  yards  long,  and 
others  are  the  demies  or  petites  decharges  of  the  French  voyageurs,  where 
it  is  only  necessary  to  lighten  the  canoe  in  order  to  float  with  safety  over 
the  bowlders. 

Differences  in  water  level— The  differences  in  level  of  the  bodies  of 
water  in  the  district  are  very  considerable;  but,  owing  to  the  fact  that  these 
differences  are  rarely  shown  by  bodies  of  water  near  one  another,  there  are  no 
high  falls.  The  highest  lake  above  the  level  of  the  sea  is  a  small  one  very 
near  the  top  of  one  of  the  highest  hills  in  sec.  20,  T.  65  N.,  R.  4  W.,  in  the 
eastern  part  of  the  district.  This  is  at  an  elevation  of  1,880  feet.  Bass- 
wood  Lake,  the  lowest  body  of  water  in  the  district,  is  1,300  foet  above  sea 
level.  The  difference  of  these  extremes,  separated  by  26  miles,  is  only  580 
feet. 

Water  power. — The  streams  connecting  these  lakes  have  for  the  most  part 
unimpoi-tant  rapids  in  them.  In  a  number  of  places,  however,  considerable 
water  power  can  be  developed,  as,  for  instance,  at  the  Kawashachong  Falls, 
where  the  Kawishiwi  empties  into  Fall  Lake:  at  the  Pipestone  Falls  on 
Newton  Lake;  at  several  places  on  the  upper  Kawishiwi  in  sees.  30,  28, 


DRAINAGE.  43 

and  24,  T.  63  N.,  R  10  W.;  in  sees.  19  and  20,  T.  63  N.,  R.  9  W.;  in  sec.  1, 
T.  64  N.,  R.  9  W.,  on  Basswood  Lake;  at  the  falls  between  White  Iron  and 
Birch  lakes;  and  at  the  falls  between  White  Iron  and  Stuntz  lakes,  just 
below  the  south  edge  of  the  district.  Of  these  the  most  accessible  and  the 
best,  and  hence  the  ones  most  likely  to  be  used,  are  the  falls  of  the 
Kawashachong,  with  a  fall  of  about  32  feet,  and  Pipestone  Falls,  and  the 
falls  between  White  Iron  and  Birch  lakes.  By  a  small  dam  at  the  outlet 
of  many  of  the  lakes,  large  reservoirs  could  be  formed  and  considerable 
water  power  developed  at  little  cost. 

Origin  of  the  lakes. — General  statements  are  very  frequently  made  con- 
cerning the  lake  regions  of  the  Northern  States  lying  within  the  limits  of 
the  glacial  drift,  especially  of  the  lake  regions  of  Wisconsin  and  Minnesota, 
which  would  lead  the  casual  reader  to  suppose  that  all  of  the  lakes  in  these 
regions  owe  their  origin  solely  to  the  agency  of  the  drift,  i.  e.,  that  they  are 
mere  depressions  within  the  general  drift  mantle  which  have  been  filled  with 
water.  This  is  certainh"  true  for  a  great  number  of  the  lakes  in  the  drift- 
covered  portion  of  North  America.  In  the  case  of  the  Vermilion  district  of 
Minnesota,  however,  this  simjjle  mode  of  origin  can  be  predicated  of  but 
few  of  the  lakes,  and  those  ai-e  all  small.  The  greater  number  of  the  lakes 
have  had  a  mixed  mode  of  origin ;  they  owe  their  existence  to  pre-Glacial 
erosion,  which  scooped  out  deep  valleys,  and  then  to  the  drift,  which  left 
dams  across  these  valleys  at  intervals.  It  is  very  probable  that  glacial 
erosion  was  also  active  in  widening  and  deepening  these  pre-Glacial 
valleys,  changing  V-shaped  into  U-shaped  valleys.  Many  of  the  lakes 
empty  over  rocky  rims.  They  occupy  basins  formed  by  the  damming  of 
pre-Glacial  valleys  by  drift,  and  their  present  outlets  are  higher  than  the 
original  mouth  of  the  valley.  However,  the  writer  nowhere  observed 
rock-basin  lakes  which  he  could  interpret  as  due  to  glacial  erosion. 

The  lakes  in  the  western  part  of  the  district  can  be  readily-  divided  into 
those  which  owe  their  present  location  and  existence  solelv  to  glacial  action, 
and  those  which  owe  their  existence  to  Pleistocene  glaciation,  but  whose 
present  location  and  configuration  are  chiefly  due  to  the  geologic  structure 
of  the  pre-Glacial  rocks  and  to  pre-Glacial  drainage.  To  the  first  kind 
belong,  among  others,  the  oval  or  irregular  lakes  lying  in  the  deep 
morainal  di'ift  which  stretches  northeast-southwest  through  T.  61  N.,  R.  14 
W^,  and  T.  62  N.,  R.  13  ^Y. 


44  THE  VERMILION  IRON-BEARING  DISTRICT. 

Lake  Vermilion  is  a  good  illustration  of  tlie  second  kind  of"  lake.  Its 
very  irregular  outline,  with  its  islands  and  bays,  is  due  chiefly  to  the  geo- 
logic structure  and  differential  erosion  of  the  various  closelv  folded  rocks 
touching  its  shores.  The  reader  is  referred  to  the  statement  on  page  432, 
wherein  attention  is  called  to  the  fact  that  in  many  cases  the  islands  are  the 
crests  of  anticlines  of  harder  rocks,  the  basins  between  being  in  the  slate 
synclines;  and  also  to  the  statement  that  the  large  bays  on  the  east  end  of 
the  lake  are  found  invariably  to  be  in  the  younger  Lower  Huronian  rocks. 
However,  even  in  this  western  part  of  the  district  where  the  drift  is  rela- 
tively heavy,  the  general  trend  of  the  long  direction  of  the  lakes  corresponds 
to  the  trend  of  the  structural  features;  that  is,  it  is  about  N.  60°-80°  E. 
These  lakes  in  the  western  portion  of  the  district  have  relatively  large 
drainage  basins,  and  are  usually  bordered  by  low  shores  clothed  with  small 
second-growth  timber.  Near  these  shores  and  back  from  them  within  their 
drainage  basins  one  very  commonly  finds  swamps  of  considerable  extent, 
which  are  not  very  much  above  the  lake  level.  The  water  of  the  lakes 
is  clear  but  is  almost  invariably  tinged  by  the  coloring  matter  brought  in 
from  these  tributary  swamps.  This  coloring  varies  nuich  in  intensity  in  the 
different  lakes,  and  although  the  writer's  observations  extended  over  onl}'  a 
portion  of  the  year — the  months  of  July,  August,  September,  and  into 
October — it  was  very  noticeable  that  the  intensity  of  the  coloring  varied 
in  the  same  body  of  water,  being  less  in  the  late  fall,  when  the  water  was 
low,  than  in  the  early  summer,  just  after  the  heavy  rains,  when  the  swamps 
and  streams  were  flooded.  Sometimes  the  organic  coloring  matter  is  so 
plentiful  that  a  bucketful  of  the  water  shows  a  decided  brown  color.  Such 
waters,  although  clear,  are  not  very  transparent.  It  is  almost  impossible  at 
times  to  distinguish  dark  bodies  6  inches  below  the  surface  of  such  water. 
Canoeing  in  smooth  water  of  this  nature  is  somewhat  hazardous;  the  bow- 
man, even  when  keeping  a  .sharp  lookout,  can  scarcely  see  the  reefs  and 
snags  in  the  water  until  he  is  upon  them,  whereas  in  rough  water  their 
presence  is  shown  by  the  way  in  which  the  water  breaks  on  them. 

The  lakes  in  the  eastern  part  of  the  district  oifer  a  striking  contrast  to 
those  in  the  western  portion  which  have  just  been  described.  They  are, 
with  the  exception  of  some  of  the  largest  lakes,  almost  uniformly  long  and 
narrow,  are  surrounded  by  high  and  bare  rocky  clitts,  and  lie  in  distinctly 
structural  basins.     Later  a  number  of  instances  will  be  cited  (p.  432)  to  show 


DRAINAGE.  45 

the  relationship  of  the  distribution  of  the  lakes  to  the  various  structural  feat- 
ures of  the  district;  here  only  two  will  be  mentioned.  The  lakes  in  sees.  ^, 
8,  and  9,  T.  65  N.,  R.  6  W.,  are  separated  by  barriers  of  glacial  drift.  The 
depression  iu  which  they  lie  was  evidently  determined  by  the  structure  of 
the  slates,  and  is  clearly  of  pre-Glacial  origin.  The  existence  of  this  string 
of  lakes  is  due  to  the  low  drift  barriers  in  which  the  connecting  streams  are 
now  cutting.  The  most  striking  instance  which  the  writer  has  observed  of 
this  relationship  of  the  lakes  to  the  structure  is  seen  in  the  string  of  lakes 
just  north  of  the  international  bovmdary  known  as  That  Mans,  This  Mans, 
Agawa,  and  the  Other  Mans  lakes.  They  lie  in  a  great  depression  in  a 
syncline  of  slates,  and,  like  those  first  mentioned,  are  separated  by  barriers 

of  drift. 

The  lakes  in  the  eastern  part  of  the  district  have,  with  a  few  exceptions, 
very  small  drainage  areas,  and  but  few  swamps  of  any  size  are  tributary 
to  them.  Consequently  the  organic  matter  which  colors  the  water  of  the 
lakes  west  is  wanting,  and  as  a  rule  the  water  is  beautifully  clear  and 
transparent. 

A  simple  sounding  apparatus  was  used  for  one  season  with  a  view  to 
o-etting  the  depths  of  the  lake  basins.  This  apparatus  consisted  of  an  oiled 
silk  fishing  line,  with  knots  1  meter  apart,  wound  on  a  large  reel,  with  a 
three-fourths  pound  lead  plumb  bob  attached  to  the  free  end.  The  reel 
was  screwed  to  an  arm  of  light  wood.  One  end  of  this  arm  was  fastened  to 
a  crossbar  in  the  bow  of  the  canoe;  the  other  end,  on  which  the  reel  was 
screwed,  was  free  to  swing.  When  the  reel  was  not  in  use  this  arm  lay 
close  to  the  left  side  of  the  canoe  and  was  suspended  from  a  hook,  which 
kept  the  reel  in  place  and  prevented  it  from  unwinding.  When  the  reel 
w^as  to  be  used  the  arm  w^as  swung  in  front  and  to  the  right  of  the  man  in 
the  bow,  and  rested  on  the  gunwale  of  the  canoe.  The  reel  was  thus  sus- 
pended over  the  water,  and  soundings  could  readily  be  taken  and  the 
approximate  deptlis  read  by  counting  the  knots  and  estimating  the  fractions 
of  meters.  By  this  means  soundings  were  taken  in  a  number  of  the  lakes. 
These  showed  that  the  lakes  in  the  western  part  were  shallow.  For 
instance,  Lake  Vermilion,  the  largest  body  of  water  in  the  district,  gave  iu 
two  places  a  depth  of  10  meters.  The  average  depth  of  the  lake  would  be 
much  nearer  6  meters.  In  contrast  to  this  the  narrow,  clear-water  lakes  in 
the  eastern  portion  of  the   district,  those  with  high,  rocky   shores,    were 


-!6  THE  VERMILION  IRON-BEARING  DISTRICT. 

found  to  be  deep.  These  are  the  lakes  in  which  tlie  trout  and  bass  are 
most  abundant  and  shoAV  best  their  fighting  quahties.  A  maximum  depth 
of  G0|  meters  (199  feet)  was  found  near  the  center  of  Lake  Gobbemichi- 
ijamnia.  The  writer  did  not  have  a  chance  to  sound  in  some  of  the  other 
lakes  of  the  district,  in  which,  if  one  may  judge  from  the  character  of  the 
water  and  of  the  surrounding  shores,  even  greater  depths  would  be  found. 
The  soundings  taken  are  recorded  on  the  accompanying  maps.  The  points 
at  which  the  soundings  were  made  were  located  approximately  by  a  rough 
system  of  triangulation,  and  are  indicated  on  the  maps  by  dots,  adjacent  to 
which  are  placed  the  figures  giving  the  depth  in  meters. 

PlanUon. — During  the  season  of  1899  a  seining  apparatus  was  carried 
and  collections  of  the  plankton  of  thirty  of  the  lakes  of  the  Vermilion  dis- 
trict were  made.  The  lakes  from  wliich  the  specimens  were  taken  were 
scattered  from  the  western  to  the  eastern  part  of  the  district,  and,  it  would 
seem,  should  give  a  fair  idea  of  the  general  character  of  the  plankton  of 
this  district.  These  collections  were  given  to  Dr.  E.  A.  Birge,  of  the 
University-  of  Wisconsin,  for  study.  He  reports  as  follows:  '-The  col- 
lections made  by  Mr.  Clements  have  been  examined.  They  contain  very 
few  Crustacea  and  no  species  except  those  whose  presence  in  these  lakes 
would  be  a  matter  of  course,  since  they  belong  to  genera  and  species  very 
widely  distributed  on  this  continent.  In  view  of  these  facts,  a  more 
detailed  report  does  not  seem  advisable." 

EXPOSURES. 

In  spite  of  the  glacial  drift,  the  rocks  are  very  well  exposed.  This 
is  due  to  the  fact  that,  as  before  stated,  the  drift  was  originalh'  not  very 
thick,  and  tliat  since  its  deposition  it  has  been  considerably  removed.  It 
is  also  due  to  the  presence  of  the  great  number  of  lakes,  excellent  exposures 
()f  the  rocks  a])pearing  around  their  shores  and  upon  the  small,  rocky  islands 
dotting  tlieir  surfaces.  For  example,  within  the  immediate  vicinity  of 
^'ermi]ion  Lake,  on  the  islands  within  this  lake,  and  around  its  shores, 
wh(-re  drainage  has  been  especially  effective,  the  rocks  rise  up  hi  bare 
liills.  The  eastern  part  of  tlie  district  beyond  Moose  Lake  contains  also 
vast  areas  in  wliich  clean  rock  surfaces  are  exposed  nearly  everywhere, 
;ni(l  uianv  of  these  exposiu'es  are  of  very  large  size.  Between  Tower  and 
xMoose  Lake  there  are  a  number  of  square  miles  in  which  the  drift  deposits 


FORESTS.  47 

are  so  deep — this  is  especially  true  in  the  area  overlain  by  the  Vermilion 
moraine — that  but  few  exposures  could  be  found.  Moreover,  within  this 
area  of  deep  drift  the  forest  growth  is  especially  luxuriant,  and  this  tends 
to  conceal  those  exposures  that  do  exist.  As  a  consequence,  the  difficulty  of 
determining  the  structure  of  these  areas  is  greatly  increased,  and  the  results 
are  less  reliable  than  for  other  portions  of  the  district. 

FORESTS. 

With  respect  to  the  forests  also  the  Vermilion  district  may  be  divided 
into  two  contrasting  areas,  a  western  and  an  eastern.  These  areas  are 
separated  approximately  by  a  line  drawn  south  from  the  international 
boundary  at  the  western  end  of  Knife  Lake,  through  the  eastern  end  of 
Ensign  or  Iron  Mountain  Lake,  across  the  north  side  of  Snowbank  Lake  to 
Moose  Lake,  and  then  through  the  eastern  end  of  North  Twin  (or  North 
Triangle)  Lake  to  the  Kawishiwi  River. 

The  western  area  is  to  a  considerable  extent  heavily  .wooded  with  old 
forests  of  mixed  growth.  On  the  whole,  the  hard  wood,,  especially  birch, 
seems  to  predominate ;  but  scattered  through  the  hard  wood  there  are  large 
areas  of  white  pine  (Pinus  strohus)  and  Norway  or  red  pine  {Pinus  resinosa). 
The  value  of  these  forests  at  present  is  chiefly  due  to  these  conifers.  With 
the  birch  are  found  some  jjoplars  and  scattering  soft  maples,  jack  pines 
black  pine  (Pinus  hanksiana),  spruce,  and  balsam  fir.  Tamarack  (hackma- 
tack, or  American  larch)  and  white  cedar  (arbor  vitae)  are  also  present  in 
varying  quantity.  The  undergrowth  consists  of  smaller  birch  and  poplar, 
soft  maple,  mountain  ash,  black  ash,  willow,  alder,  hazel,  pin  and  choke 
cherry,  jack  pine,  balsam  fir,  spruce  (the  last  two  in  places  forming 
almost  impenetrable  thickets),  ground  hemlock,  the  high-bush  cranberry, 
a  viburnum  (Vihurnmn  ojndus),  June  berry  or  service  berry  (Amelanchier 
canadensis),  and  some  other  less  important  kinds.  In  some  portions  of  this 
western  area,  especially  south  of  Eagle  Nest  Lakes,  southeast  of  Fall  Lake, 
near  the  North  and  South  Twin  lakes,  and  south  and  west  of  Pine  Lake, 
the  country  takes  on  the  aspect  of  a  true  pinery.  If  may  be  noted  here 
that  the  above-mentioned  areas  are  the  ones  in  which  the  drift  is  especially 
heavy.  Thus  one  may  see  the  intimate  relationship  existing  between  the 
geology  of  the  district  and  its  forest  growth.  In  these  areas  red  and  Avhite 
pine  is  the  chief  growth,  with  the  former  rather  in  the  ascendency.     There  is 


48  THE  VERMILION  IRON-BEARING  DISTRICT. 

very  little  uudergTOwth  in  these  places,  and  this  is  chieflj'  cherry,  balsam, 
spruce,  and  ground  hemlock.  Extensive  lumbering  operations  are  carried 
on  in  these  pineries,  and  in  a  verj^  few  years  the  pine  will  have  been  cut 
from  most  of  the  large  tracts.  It  will  then  probably  be  l^ut  a  year  or  two  at 
the  most  before  fire  will  get  into  the  old  pine  slashings,  and  any  isolated 
uncut  tracts  will  therebj'  be  destroyed,  as  will  also  adjacent  hardwood  areas. 
Scattered  through  this  western  area  are  larg-e  tracts  which  have  been 
biu'ued  over  one  or  more  times  within  the  last  ten  to  twenty  years.  In 
some  places  the  fire  was  so  severe  as  to  destroy  the  humus  as  well  as  the 
timber.  As  a  consequence  of  the  removal  of  these  protections,  the  major 
portion  of  the  soil,  and  even  in  some  cases  the  subsoil,  has  been  washed 
into  the  valleys,  and  the  hills  are  now  practically  bare  rock.  Such  an  area 
is  that  known  as  the  "Burned  Forties,"  in  sees.  23  and  24,  T.  62  N.,  R.  15  W. 
Where  the  soilonly  was  removed  it  has  required  some  time  for  the  snbsoil 
to  reach  a  condition  suitable  for  plant  growth,  and  the  hills  in  snch  areas 
are  covered  only  with  grass,  weeds,  and  stunted  poplars,  birch,  and  jack 
pine.  In  other  areas  the  fire  occurred  so  long  ago  that  sufficient  soil  h<is 
accnmulated  to  support  a  dense  growth  of  poplar,  birch,  and  jack  pine, 
which  has  reached  fair  size.  In  some  places  in  such  burned  areas  the 
second  growth  is  almost  exclusively  poplar;  in  other  localities  the  jack 
pine  or  birch  may  predominate.  The  usual  history  of  such  an  area 
after  it  has  been  burned  is  as  follows:  The  year  after  the  fire  has  run 
through  the  forest  there  is  always  a  heavy  growth  of  fireweed  (so  called  in 
that  region) — mare's  tail — which  springs  np.  This  is  soon  succeeded  by 
pojjlar,  cherry,  bircli,  jack  pine,  and  rarely  seedling  white  and  Norwaj^ 
Ijine.  As  a  result  of  the  deadening  of  the  original  forest  trees  and  their 
consequent  weakening,  they  very  readily  succumb  to  the  strong  winds. 
They  are  blown  down,  and  this  fallen  timber,  with  the  dense  second  growtli 
that  springs  up  between  the  recumbent  trunks,  renders  such  areas  extremely 
difficult  to  tra\'erse.  Not  many  years  elapse  before  this  second  growth  1-= 
swept  by  fire,  and  in  its  turn  falls  and  is  replaced  by  a  third  growth.  The 
repetition  of  such  occurrences  renders  it  increasingly  diflicult  to  traverse 
such  burned  country  uidess  the  fire  has  been  very  recent  and  of  suflicient 
intensity  to  destroy  completely  both  standing  and  fallen  timber.  An 
occasional  rotted  and  partly  liurned  log  of  large  size  in  the  midst  of  the 
pines  seems  to  indicate  that  long  ago  fires  ran  through  even  tliose  areas  in 


FORESTS.  49 

which  the  present  forests  are  frequentl}'  spoken  of  as  the  original  growth^ 
and  destroyed  an  earher  forest  then  existing. 

This  northern  country  offers  obstructions  to  the  explorer  such  as  can 
probably  be  met  -with  elsewhere  onl}-  in  tropical  countries.  It  is  compara- 
tively easy  to  travel  thi-ough  the  forests  of  standing  Norway  and  white 
pine,  for  here  one  finds  but  sparse  undergrowth ;  but  only  a  very  small  part 
of  the  district  is  covered  by  such  open  forest;  the  greater  portion,  especially 
in  this  western  part,  is  covered  by  exceedingly  dense  forests  of  birch, 
balsam,  and  jack  pine,  with  undergrowth  that  is  almost  impenetrable  in 
places.  Between  the  areas  of  high  ground  covered  with  the  above- 
mentioned  forest  growth  there  lie  some  swampy  areas  of  tamarack  and 
cedar  and  open  muskegs.  During  wet  years,  many  of  these  swamps  are 
flooded,  so  that  in  crossing  them  one  wades  in  water  2  to  3  feet  deep. 
Windfalls  have  destroyed  vast  patches  of  timber  and  have  left  the  trunks 
piled  upon  one  another  in  inextricable  confusion,  and  a  second  growth  in 
places  adds  further  to  the  entanglement  and  increases  the  difficulties  of 
the  traveler.  One  inexperienced  in  a  country  of  this  character  would  feel 
that  the  task  were  well-nigh  hopeless  were  he  called  upon  to  leave  the 
canoe  routes  and  beaten  trails  and  explore  this  wilderness.  It  sometimes 
requires  two  hours  to  advance  a  mile,  and  to  run  a  line  5  miles  in  length 
and  explore  the  area  for  a  few  hundred  yards  on  both  sides  is  a  good  day's 
work. 

In  this  western  jjortion  of  the  district  there  are  a  number  of  very 
extensive  wild  cranberry  marshes  and  other  marshes  that  would  be  suitable 
for  the  cultivation  of  cranberries.  There  seems,  indeed,  to  be  no  good 
reason  why  these  marshes  should  not  be  improved  and  cranberries  grown 
upon  them  for  the  market.  In  other  States  such  marshes  have  proved  a 
good  investment,  and  it  would  seem  that  a  good  opportunity  for  their 
development  is  offered  in  this  district. 

The  eastern  half  of  the  Vermilion  district  may  be  spoken  of  as  the 
burned  area.  In  it  there  are  but  a  few  isolated  and  very  small  patches 
of  large  timber.  This  portion  of  the  district  seems  to  have  been  frequently 
swept  by  fires,  and  at  present  the  growth  covering  it  is,  with  few  exceptions, 
very  small.  It  is  probable  that  the  character  of  the  ground  has  been  a  promi- 
nent factor  in  determining  the  size  of  the  second  growth.  This  portion  of 
the  district  as  a  whole  is  very  rocky,  and  the  drift  and  the  soil  are  much 
MON  XLV — 03 4 


50  THE  VERMILION  IRON-BEARING  DISTRICT. 

thinner  than  to  the  Avest.  The  timber  is  the  same  as  that  which  occurs 
farther  west,  with  the  ditference  that,  since  it  is  nearly  all  second  growth, 
poplar,  jack  jiine,  and  birch  predominate,  in  the  order  given.  In  some 
places  within  this  area  the  fire  has  been  so  intense  that  even  the  swamp 
growth  has  been  destroyed,  and  in  place  of  the  original  cedar  swamps  we 
now  find  grassy  meadows.  There  is  no  way  of  determining  accm-ately 
just  when  this  area  was  denuded  of  its  forests.  On  the  Government  plats 
there  is  a  note  to  the  efi^ect  that  the  country  near  Gunflint  Lake  w^as  burned 
over  in  the  sixties  In  some  ijlaces  the  section  corners  are  marked  on 
second-growth  trees,  showing  that  the  burning  took  place  a  number  of 
years  prior  to  the  time  that  the  region  was  surveyed.  In  other  places  the 
second-growth  birch  is  at  least  twenty-  years  old,  and  here  no  survey  lines 
of  any  kind  were  to  be  found.  On  the  other  hand,  there  is  abundant  evi- 
dence that  fires  liaA^e  run  over  portions  of  the  area  since  it  was  surveyed, 
for  large  trees  with  the  marks  of  the  corners  and  quarterposts,  and  the  bear- 
ing trees  Avith  their  marks  on  them,  haA^e  been  scorched  since  these  marks 
were  made;  and,  indeed,  in  many  cases,  the  marks  themselves  have  been 
nearh'  obliterated. 

SOIL. 

The  soil  throughout  tlie  district  is  thin,  but  what  there  is,  being  of 
glacial  origin,  is  of  A-ery  good  character  and  lends  itself  readily  to  cultiva- 
tion. In  the  A'alleys  the  soil  has  accumulated  in  places  to  considerable 
depth,  and  where  some  of  the  swamps  have  been  drained  and  properly 
treated  the  crops  produced  are  excellent.  However,  farming  is  injured  by 
the  climatic  conditions,  which  are  unfavorable  to  the  groAvth  and  maturity 
of  all  but  a  feAv  crops.  Hay  can  be  successfully  raised.  Potatoes,  cab- 
bage, and  rutabagas  of  excellent  quality  can  also  be  grown,  and  all  of  these, 
especially  the  hay,  bring  good  prices.  Suitable  land  for  farming  on  a 
large  scale  is  found  in  but  fcAV  places.  On  some  natural  meadows  in  dried 
lake  basins  and  along  the  margins  of  the  streams  and  lakes  good  crops  of 
hay  are  made. 

GAME  AND   IISH. 

This  portion  of  Minnesota  is  fairly  Avell  stocked  Avith  game.  Moose, 
deer,  and  bear  Avere  seen  repeatedly.  In  many  places  the  swamps  are  trav- 
ersed by  deep  cut,  recent  moose  trails,  and  occasionally  there  were  found 
small  areas  so  tramped  and  torn  bA-  these  animals  as  to  resemble  a  cattle 


GAME  AND  FISH.  51 

yard.  Moose  must  be  fairly  abundant,  therefore,  although  no  great  num- 
ber were  seen,  this  being-  due  chiefly  to  the  fact  that  the  party  made  no 
attempt  to  go  quietly  through  the  woods.  The  animals  were  frequently 
heard  crashing  through  the  underbrush.  Only  one  caribou  was  seen, 
though  in  portions  of  the  district  their  tracks  and  runways  were  common. 
Pickerel,"  wall-eyed  pike,'  bass,  the  namaycush  or  lake  trout,  and  white- 
fish  are  the  kinds  of  fish  most  used  for  food,  and  occur  in  abundance,  about 
in  the  order  given.  Many  of  the  lakes  are  teeming  with  fish,  but  they  are 
usually  the  least  desirable  kind,  pickerel  and  perch.  These  can  be  obtained 
in  all  of  the  lakes,  and  they  are  so  common  and  relatively  such  a  poor  g-ame 
and  food  fish  that  the  fisherman  ordinarily  throws  them  back  into  the  lake 
with  disdain.  Usually,  however,  he  first  kills  the  pickerel,  as  they  are 
the  recognized  enemy  of  the  game  fish.  Pike  are  not  so  abundant  as 
pickerel,  but  they  are  found  in  most  of  the  lakes.  Bass  occur  in  only 
a  few,  but  where  found  they  are  in  fairly  large  numbers.  Trout 
(Salvelhms  namaycush)  are  confined  almost  exclusively  to  the  deep  lakes 
in  which  the  water  is  uncolored,  although  they  by  no  means  occur  in  all 
such  lakes.  Since  these  conditions  are  most  commonly  fulfilled  in 
the  eastern  portion  of  the  district  and  in  the  lakes  along  the  international 
boundary  and  just  across  the  boundary  on  Hunters  Island,  the  trout  are 
most  common  in  the  eastern  portion  of  the  Vermilion  district.  In  excep- 
tional cases  lake  trout  were  found  in  some  of  the  lakes  with  colored 
water — for  example,  Ogishke  Muncie — and  these  were  slightly  diflferent 
from  the  trout  in  the  lakes  with  uncolored  water.  They  are  considerably 
darker  in  color  and  appear  to  have  proportionally  heavier  bodies  and 
smaller  heads.  They  give  the  impression  of  being  a  heavier  and  slower 
fish  In  the  streams  and  lakes  from  Peter  Lake  east  to  Fay  (Paulsons) 
Lake  trout  were  caught  which  seemed  slightly  different  from  the  normal 
lake  trout      They  are  called  mountain  trout   by  the   woodsmen.     They 


<•'  Pickerel  is  the  name  commonly  applied  to  the  true  pike  (Esox lucius)  throughout  this  State,  as 
■well  as  in  Wisconsin.  It  is  easily  discriminated  from  the  wall-eyed  pike  by  its  shovel-shaped  nose  and 
the  light  spots  on  the  dark  background  of  the  body.  It  is  a  fish  which  lives  in  sluggish  waters,  among 
the  weeds,  and  very  near  the  surface  of  the  water  generally.  It  is  very  slimy  and  has  a  disagreeable, 
strong,  fishy  odoi.     The  flesh  is  soft  in  summer. 

''The  wail-eyed  pike  (Sihostedion  vitreum) ,  or  pickerel,  as  it  is  sometimes  called  in  this  region — 
sometimes  dory  and  jack-fish— is  an  excellent  food  fish,  with  firm,  well-flavored,  white  fiesh.  It  has 
a  golden-yellow  color  on  the  sides  of  the  belly,  grading  up  into  the  darker  color  of  the  back,  with  dark 
mottlings.     Tnese  mottlmgs  also  occur  on  the  fins. 


52  THE  VERMILION  IRON-BEARING  DISTRICT. 

were  uot  carefully  studied,  so  uo  g'ood  detailed  description  can  be  giveu  of 
tliem.  Ther  seem  from  general  appearance  to  be  more  nearly  like  the 
ordinary  speckled  brook  trout  (Salvelinus  fontinalis).  Some  of  the  markings 
on  the  brownish  back  resemble  those  on  the  speckled  trout,  and  they  have 
crimson  spots  on  their  sides,  but  they  ai"e  in  other  respects  different  from 
these.  This  is  probably  one  of  the  numerous  ^-arieties  of  the  lake  trout. 
According  to  the  repeated  experience  of  members  of  the  party  who,  for 
three  diflPerent  seasons  and  at  different  times  during  those  seasons,  had 
fished  in  the  same  lakes,  the  fish  caught  in  the  same  lake  usually  run  about 
the  same  size,  showing  very  slight  variations  indeed. 

White-fish  {Coregonus  clupeiformis)  are  abundant  in  a  number  of  the 
lakes,  but  since  they  are  caught  only  in  nets  they  are  not  to  be  considered 
by  the  sportsman,  although  they  are  very  important  and  very  delicious  as 
food.  They  have  been  netted  on  Basswood  for  many  years,  and  shipped 
to  southern  ^Minnesota  markets.  They  also  occur  in  Vermilion,  Saganaga, 
Knife,  Otter  Track,  Ogishke  Muncie,  and  other  lakes. 

CTJIiTURB. 

There  are  four  towns  in  the  Vermilion  district — Tower,  Saudau,  Ely, 
and  Winten.  Tower  is  the  westernmost  town  of  the  district,  and  is  situated 
on  Vermilion  Lake.  It  was  settled  in  1882  (at  that  time  there  was  one  Lig 
cabin  there),  and  according  to  the  Twelfth  Census  (1900)  has  1,366  inhab- 
itants. These  depend  almost  exclusively  for  employment  upon  lumbering- 
operations,  a  sawmill,  and  the  mines  of  Soudan.  All  of  the  stores  and 
saloons  of  this  portion  of  the  district  are  located  in  Tower,  and  they  supply 
the  people  of  Soudan  as  well  as  the  people  within  the  Tower  limits. 

Soudan,  an  unincorporated  place,  is  2  miles  northeast  of  Tower,  at  the 
foot  of  Soudan  Hill.  It  has  grown  up  around  the  Minnesota  group  of 
mines,  and  has  about  1,000  inhabitants.  It  is  essentially  a  mining  town, 
and  most  of  the  people  are  recent  immigrants  with  American-born  children. 
The  population  of  the  toAvn  consists  entirely  of  employees  of  the  Minne- 
sijta  Iron  Company  and  their  families.  The  company  allows  no  stores  or 
saloons  here. 

Ely  is  situated  about  midway  the  district,  on  the  south  shore  of  Long 
Lake.  It  is  the  most  prosperous  town  on  the  range.  It  has  3,717  inhab- 
itants, who  are  for  the  most  part  employees  of  the  Minnesota  Iron  Company 


CULTURE.  53 

and  their  families;  iu  addition  there  are,  of  course,  a  relatively  few  people 
who  are  employed  in  the  usual  stores  and  small  industries  of  various  kinds 
which  are  essential  to  the  life  of  a  town  of  this  size. 

Winteu  is  a  small  village  at  the  west  end  of  Fall  Lake.  It  is  the 
eastern  terminus  of  the  Duluth  and  Iron  Range  Railroad.  It  owes  its 
existence  to  the  presence  of  two  thriving  sawmills,  which  are  rapidly  cut- 
ting all  of  the  timber  of  this  part  of  the  district.  There  are  about  500 
people  within  a  radius  of  a  mile  from  the  mills  at  Winten. 

The  name  Silver  City  is  not  infrequently  employed  in  the  conversa- 
tion of  explorers  and  travelers  ai'ound  Ely,  and  may  lead  to  confusion  in 
the  mind  of  the  stranger.  The  name  is  applied  to  the  site  of  an  old 
exploration  which,  iu  the  sanguine  owner's  eyes,  was  the  nucleus  around 
which  there  was  to  be  developed  a  city  of  importance.  Nothing  exists 
there  now;  in  fact,  the  writer  does  not  know  that  a  single  house  was  ever 
built  there.  The  location  is  at  the  White  Iron  Lake  portage,  in  sec.  32, 
T.  63  N.,  R.  11  W.,  and  being  on  one  of  the  canoe  routes  and  frequently 
used  as  a  camping  place,  the  name  is  still  current  among  the  woodsmen. 

The  towns  just  mentioned  are  connected  by  the  Duluth  and  Iron 
Range  Railroad,  which  is  the  only  one  that  at  present  gives  service  in  the 
Vermilion  district.  Consequently  this  road  handles  all  of  the  lumber  and 
ore  that  is  shipped.  The  eastern  end  of  the  district  is  touched  by  the 
Duluth,  Port  Arthur  and  Western  Railroad,  which  was  projected  to  connect 
Port  Arthur  and  Duluth.  This  road  was  built  from  Port  Arthur  as  far 
west  as  Paulson's  mine  at  Gruuflint,  in  sec.  28,  T.  65  N.,  R.  4  W.  There 
are  a  few  houses  here,  but  since  the  abandonment  of  the  mine,  no  inhab- 
itants. The  two  termini,  Paulson's  mine  on  the  east  and  Winten  on  the 
west,  have,  however,  never  been  connected  except  by  the  railroad  survey. 
At  present  the  road  within  the  United  States  for  the  6  miles  from  the 
boundary  to  Paulson's  is  impassable,  the  trestles  and  many  ties  having  been 
burned.  It  would  require  extensive  rebuilding  before  it  could  be  used, 
and  if  rebuilt  a  new  route  for  a  part  of  the  way  should  certainly  be 
selected,  as  it  would  be  almost  impossible  to  lay  out  any  route  that  would 
not  be  an  improvement  over  some  23arts  of  its  pi'esent  location.  Wagon 
roads  throug-hout  the  district  are  few  in  number.  Those  near  the  towns 
are  kept  iu  fair  condition,  but  elsewhere  they  are  very  poor.  Even  the 
county  road  between  Soudan  and  Ely  is  poorly  kept.     Very  few  branches 


54  THE  VERMILION  IRON-BEARING  DISTRICT. 

run  off  from  this  road,  so  that  the  country  to  the  north  and  south  of  it 
must  be  reached  by  means  of  the  few  homesteaders'  trails  that  exist,  or 
else  by  tramping  through  the  woods.  East  of  AVinten  the  traveler  can 
proceed  in  summer  only  on  foot  and  by  means  of  canoes. 

INDIAN  RESERVATION. 

Jn  Sucker  Point,  a  large  point  of  land  projecting  northeast  into 
Vermilion  Lake  just  across  the  bay  from  the  mill  at  Tower,  there  is  a 
reservation  which  is  occupied  by  the  Bois  Fort  band  of  the  Ojibwa  or 
Chippewa  Indians.  According  to  the  last  report  of  the  Commissioner  of 
Indian  Affairs,  there  were  808  Indians  living  on  this  reservation  on  June 
30,  1900.  Of  these,  however,  a  considerable  number  really  live  outside  of 
the  reservation,  many  of  them  being  located  in  the  vicinity  of  Ely.  The 
Indians  are  found  in  large  numbers  near  the  reservation  only  about  the 
Fourth  of  July,  and  at  the  times  when  the  regular  Grovemment  payments 
are  made.  During  the  winter  they  are  widely  scattered  over  the  country, 
hunting  and  trapping,  and  in  summer  are  encamped  on  the  shores  of 
the  lakes,  where  fish  and  berries  are  abundant.  In  1898  the  Government 
selected  a  location  on  Sucker  Point  and  erected  thereon  a  number  of 
commodious  buildings  to  be  used  for  dwellings  for  the  teachers  and  Indian 
children  and  for  school  purposes,  but  these  Indians  are  not  progressive,  and 
do  not  take  kindly  to  the  advantages  offered  them  by  the  Government  to 
become  educated  agriculturists,  or  otherwise  good  citizens.  They  are  very 
apt  in  acquiring  the  vices  of  ci^dlization;  and  instead  of  cultivating  the 
available  land  on  their  reservation,  they  prefer  to  gain  a  precarious  liveli- 
hood by  hunting,  fishing,  and  trapping.  Only  a  very  small  portion  of  the 
arable  land  on  the  point  is  cultivated. 


CHAPTER   II. 


RESUME  OF  LITERATURE. 


The  Vermilion  district  lias  been  studied  from  a  geologic  point  of  view 
only  since  the  first  quarter  of  the  nineteenth  century.  A  number  of  years 
before  this,  howevei-,  portions  of  it  had  been  visited  by  fur  traders.  The 
well-known  international  boundary  canoe  route  passed  part  way  along  its 
northeast  and  northern  boundary,  and  has  been  used  since  time  immemorial. 
Some  of  the  early  fur  traders  and  explorers  kept  journals  of  their  travels, 
and  these  make  mention  of  this  route.  The  first  few  pages  of  this 
chapter  are  devoted  to  a  very  brief  description  of  the  main  canoe  routes 
of  the  district,  and  of  the  methods  of  travel  and  customs  of  the  fur  traders 
along  the  boundary  route  when  this  northwest  country  was  first  opened. 
The  remainder  of  the  chapter  consists  chiefly  of  abstracts  of  articles  dealing 
with  the  geologic  character  of  the  Vermilion  district.  In  this  abstract  the 
author  of  each  paper  has  been  allowed,  in  most  cases,  to  speak  for  himself. 
Where,  for  various  reasons,  this  was  not  considered  best,  the  attempt  has 
been  made  in  every  case  to  give  exactly  the  author's  meaning,  although 
his  precise  words  may  not  be  used.  While  innumerable  details  have  been 
of  necessity  omitted,  it  is  believed  that  the  salient  points  of  the  papers 
reviewed  have  been  noted  and  that  each  author's  views  have  been  correctly 
represented.  Should  it  be  found  in  any  case  that  the  author's  views  have 
not  been  correctly  stated,  the  fault  is  due  to  error  in  interpretation.  In 
statins'  the  views  of  the  various  writers,  comments  are  for  the  most  part 
refrained  from,  as  discussions  of  their  statements  will  be  found  at  the  proper 
places  in  the  descriptive  part  of  the  monograph. 

Throughout  the  abstracts  the  writer  has  followed  the  spelling  of  the 
proper  names  given  in  the  original  article.  The  great  variation  in  the 
spelling  of  these  names  will  be  clearly  seen  if  one  follows  the  name  through 
a  luimber  of  the  reports. 

In  some  cases  a  report  or  an  article  may  have  been  preceded  by  one 
or  more  papers  discussing  the  bearing  of  some  of  the  facts  presented  in 
the  final  report.  In  such  cases  the  final  paper  has  been  abstracted,  and 
references  only  have  been  given  to  the  others. 


56  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  literature  of  the  Lake  Superior  region  was  very  fully  reviewed 
by  Prof.  C.  R  Van  Hise  in  Bulletin  No.  86  of  the  United  States  Greoloffical 
Survey,  and  that  review  has  been  continued  by  him  and  Mr.  C.  K.  Leith 
up  to  the  present  time.  The  writer  has  used  this  material  freely,  and  wishes 
to  make  acknowledgment  here  of  the  assistance  afforded  Ijy  these  reviews. 

The  abstracts  are  arranged  chronologically,  in  the  order  of  the  publi- 
cation of  the  articles  abstracted.  By  following  these  abstracts  critically 
the  reader  can  acquaint  himself  with  all  pulilished  articles  dealing  with 
the  territory.  He  can  follow  the  development  of  the  views  on  the  geology 
of  the  district,  which  is  comparatively  difficult  of  access,  and  can  see  how 
the  knowledge  concerning  it  was  increased  vear  by  year. 

HISTORY   OF  EXPLORATION^  AXD   CHARACTER   OF  THE   ROUTES. 

The  international  boundary  trends  a  little  south  of  east  and  north  of 
west,  and  forms  the  eastern,  northeastern,  and  northern  boundary  of  the 
district  for  a  total  length  of  75  miles.  From  Lake  Superior  to  Rainy  Lake 
the  international  boundary  follows  a  chain  of  rivers  and  lakes,  crossing- 
necks  of  land  at  two  places.  One  of  these,  known  as  the  Height  of 
Land,  is  between  North  and  South  lakes,  and  is  the  divide  between  the 
headwaters  of  Pigeon  River,  flowing  east  to  Lake  Superior,  and  the 
waters  flowing  west  to  Rainy  Lake  and  finally  to  Hudson  Bay.  The 
other  is  a  narrow  strip  of  land  in  sec.  24,  T.  66  N.,  R.  6  W.,  about  600 
paces  in  width  at  the  portage  that  separates  the  watei's  flowing-  northeast 
into  Saganaga  Lake  and  then  northwest  around  the  north  side  of  Hunters 
Island,  so  called,  from  the  waters  flowing  northwest  and  around  the  south 
side  of  Hunters  Island,  both  of  these  finally  uniting  in  Lac  La  Croix.  This 
naiTOw  strip  of  land,  about  600  paces  wide  at  the  portage,  is  all  that 
prevents  the  body  of  land,  with  an  area  of  approximatelj^  1,000  square 
miles — to  which  the  name  Hunters  Island  was  given  erroneously,  as  we 
now  know — from  being  in  reality  an  island  instead  of  a  peninsula. 

As  is  well  known,  the  rivers  and  chains  of  lakes  marking  the 
international  boundary  have  been  for  the  Indians  the  main  route  of  travel 
from  Lake  Superior  into  the  Northwest  from  time  immemorial.  It  was  this 
same  route  that  was  followed  by  the  fur  traders  and  e.xplorers  who  first 
earned  civilization  to  the  Indians  of  the  extreme  Northwest — a  civilization 
characterized,  when  first  presented  to  them,  by  honesty  and  good  morals  in 


RESUME  OF  LITERATURE.  57 

minimiim  amount,  and  dishonesty,  lasciviousness,  and  rum  in  maximum 
quantities.  The  Vermihou  district  is  traversed  for  its  entire  length  bv  a 
canoe  route,  which  leaves  the  intei'national  boundary  route  at  Gunflint 
Lake  and  continues  westward.  At  Vermilion  Lake  this  route  joins  a  canoe 
route  that  comes  from  Rainy  Lake,  by  way  of  Vermilion  River.  It  then 
ascends  Pike  River,  crosses  the  divide — the  Giants  range — south  of  Ver- 
milion Lake,  and  thence  continues  on  down  St.  Louis  River  to  Duluth, 
where  Lake  Superior  is  reached.  The  Vermilion  district  can  also  be 
reached  from  Lake  Superior  by  canoe  routes  from  Grand  Marais,  Beaver 
Bay,  and  other  points  on  the  lake  shore. 

The  route  along  the  boundary  is,  of  course,  well  known  to  every 
student  of  the  history  of  the  Northwest,  and  for  one  imbued  with  a  love  of 
history  as  well  as  nature  a  pleasauter  journey  can  scarcely  be  conceived 
than  that  which  can  be  so  delightfully  made  in  canoe  from  Grand  Portage, 
on  Lake  Superior,  to  Rainy  Lake,  or  farther  to  the  northwest  if  one  chooses." 

The  easternmost  part  of  the  route  is  known  by  the  name  of  Grand 
Portage.  This  name  was  at  first  applied  to  the  portage,  9  miles  long,  from 
Lake  Superior  to  Pigeon  River,  and  has  since  been  given  to  the  settle- 
ment at  the  Lake  Superior  end-  of  the  portage.  This  part  of  the  route  is 
mentioned  in  the  accounts  of  nearly  all  of  the  early  explorers,  and  if  they 
did  not  use  the  route  they  at  least  visited  or  heard  of  the  port,  as  it  was 
one  of  the  most  important  settlements  on  the  chain  of  Great  Lakes. 

The  literature  dealing  with  the  part  of  the  international  canoe  route 
that  touches  the  Vermilion  district  has  not  been  fully  examined,  but  some 
pains  have  been  taken  to  get  references  to  it,  descriptions  of  it,  and 
mode  of  travel  over  it  from  the  works  of  the  eai'ly  explorers.  Jonathan 
Carver*  mentions  the  Grand  Portage  settlement,  which  he  visited,  but 
describes  Rainy  Lake  and  the  route  to  it  only  from  hearsay.  Alexander 
Mackenzie,''  who  must  have  ti-aversed  the  region  a  number  of  times,  gives 

0  "The  route  from  Grand  Portage  to  Rainy  Lake  traverses  about  100  miles  of  diatance,  the  direct 
line  being  not  far  from  70  miles.  There  are  29  portages.  The  first,  or  'grand  portage,'  is  8}  miles, 
the  third  is  IJ,  the  ninth  is  1  j,  and  the  residue  vary  from  a  few  steps  to  a  half  mile.  The  total  of 
portages,  15  miles.  The  water  communications  are  chiefly  small  lakes  of  3  to  10  or  20  feet  deep." 
Hanchett  and  Clark:  Report  on  Geology  of  Minnesota,  pp.  47-48,  1865.  See  Vermilion  literature 
references,  p.  66  of  this  monograph. 

6  Travels  Through  the  Interior  Parts  of  North  America,  by  Jonathan  Carver,  Edition  1778,  pp. 
106-115. 

«  Voyages  from  Montreal,  on  the  River  St.  Laurence,  Through  the  Continent  of  North  America,  to 
the  Frozen  and  Pacific  Oceans,  in  the  years  1789  and  1793,  by  Alexander  Mackenzie,  London,  1801. 


58  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  best  description  of  the  route,  as  well  as  of  the  method  of  travel  over  it, 
which  the  writer  has  thus  far  found.  As  essentially  the  same  method  is  in 
use  at  the  present  day,  with  the  difference  that,  since  the  transport  of  furs 
over  this  route  is  no  longer  of  impoi'tance,  the  canoes  used  are  not  so  large 
and  the  number  of  men  employed  is  A'ery  much  smaller,  the  desciiption  of 
that  portion  of  the  route  leading  from  Lake  Superior  into  the  Vermilion 
district  seems  to  be  of  sufficient  interest  to  warrant  its  insertion  here  in  the 
author's  words,  with  the  addition  of  a  few  footnotes,  added  chiefly  for  the 
purpose  of  enabling  the  reader  to  identify  the  lakes  by  their  present  names 
with  the  lakes  as  known  to  Mackenzie.  His  description  of  the  route  is 
given  in  connection  with  his  account  of  the  rise,  progress,  and  condition  of 
the  fur  trade,  in  which  the  author  was  ioterested  as  one  of  the  partners 
of  the  Northwest  Fur  Company. 

Mackenzie's  description  of  Grand  Portage  Bay  and  its  surroundings, 
at  the  eastern  end  of  the  canoe  route,  is  very  good.  Let  us  refer  to  this 
description  and  attempt  to  see  the  bay  as  he  saw  it,  sm-rounded  by  hills 
rising  to  a  height  of  730  feet,  its  bosom  dotted  with  canoes  and  its  shores 
bearing  the  tents  and  wigwams  of  the  fur  traders  and  Lidians.  Back  from 
the  shore  and  on  the  slope  of  the  hill  was  the  fort,  which  was  occupied  bv 
the  traders  and  trusted  employees,  and  in  which  the  goods  were  stored. 
The  traders  with  their  stores  came  fi'om  ^Montreal,  but  we  shall  not  attempt 
to  follow  their  journey  in  detail. 

A  quantitv  of  their  goods  are  sent  from  Montreal  in  boats  to  Kingston,  at  the 
entrance  of  Lake  Ontario,  and  from  thence  in  vessels  to  Niagara,  then  overland  10 
miles  to  a  water  communication,  by  boats,  to  Lake  Erie,  where  they  are  again 
received  into  vessels,  and  carried  over  that  lake  up  the  river  Detroit,  through  the 
lake  and  river  Sinclair  to  Lake  Huron,  and  from  thence  to  the  Falls  of  St.  Mark's, 
when  the}'  are  again  landed  and  carried  for  a  mile  above  the  falls  and  shipped  over 
Lake  Superior  to  the  Grande  Portage  [p.  xxxix].  ...  At  length  they  all  arrive  at 
the  Grande  Portage,  which  is  160  leagues  from  St.  Marys,  and  situated  on  a  pleasant 
bay  on  the  north  side  of  the  lake.  .  .  . 

At  the  entrance  of  the  bay  is  an  island  which  screens  the  harbor  from  every  wind 
except  the  south.  The  shallowness  of  the  water,  however,  renders  it  necessary  for 
the  vessel  to  anchor  near  a  mile  from  the  shore,  where  there  is  not  more  than  14 
feet  water  [p.  xl]. 

******** 

The  bottom  of  the  ba}-,  which  forms  an  amphitheater,  is  cleared  of  wood  and 
incio-sed.  and  on  the  left  corner  of  it,  beneath  a  hill  3(i(>  or  400  feet  in  height,  and 


RESUME  OF  LITERATURE.  59 

crowned  bj'  others  of  a  still  greater  altitude,  is  the  fort,  picketed  in  with  cedar 
pallisadoes  and  inclosing  houses  built  with  wood  and  covered  with  shingles.  They 
are  calculated  for  everj^  convenience  of  trade,  as  well  as  to  accommodate  the  i3ro- 
prietors  and  clerks  during  their  short  residence  there.  The  North  men  live  under 
tents;  but  the  more  frugal  pork  eater  lodges  beneath  his  canoe.  The  soil  immediatel}' 
bordering  on  the  lake  has  not  proved  verj^  propitious.  .  .  .  There  are  meadows  in 
the  vicinit}'  that  3'ield  abundance  of  haj'  for  the  cattle;  but,  as  to  agriculture,  it  has 
not  hitherto  been  an  object  of  serious  consideration. 

I  shall  now  leave  these  geographical  notices  to  give  some  further  account  of  the 
people  from  Montreal.  When  thej'  are  arrived  at  the  Grande  Portage,  which  is 
near  9  miles  over,  each  of  them  has  to  cany  eight  packages  of  such  goods  and 
provisions  as  are  necessarj'  for  the  interior  country.  This  is  a  labor  which  cattle 
can  not  conveniently  perform  in  summer,  as  both  horses  and  oxen  ^v•ere  tried  by 
the  compan}"  without  success.  .  .  . 

Having  finished  this  toilsome  part  of  their  duty,  if  more  goods  are  necessary  to 
be  transported,  they  are  allowed  a  Spanish  dollar  for  each  package;  and  so  inured 
are  thej'-  to  this  kind  of  labor,  that  I  have  known  some  of  them  set  off  with  two 
packages  of  90  pounds  each,  and  return  with  two  others  of  the  same  weight,  in 
the  course  of  six  hours,  being  a  distance  of  18  miles  over  hills  and  mountains." 
This  necessary  part  of  the  business  being  over,  if  the  season  be  early  they  have 
some  respite,  but  this  depends  upon  the  time  the  North  men  begin  to  arrive  from 
their  winter  quarters,  which  they  commonly  do  early  in  July.  At  this  period  it  is 
necessary  to  select  from  the  pork  eaters  a  number  of  men,  among  whom  are  the 
recruits,  or  winterers,  suificient  to  man  the  North  canoes  necessary  to  carry  to  the 
river  of  the  rainy  lake  the  goods  and  provisions  requisite  for  the  Athabasca  country, 
as  the  people  of  that  country  (owing  to  the  shortness  of  the  season  and  length  of 
the  road  can  come  no  farther)  are  equipped  there,  and  exchange  ladings  with  the 
people  of  whom  we  are  speaking,  and  both  return  from  whence  they  came.*  This 
voyage  is  performed  in  the  course  of  a  month,  and  they  are  allowed  proportionable 
wages  for  their  services  [pp.  xliii-xlv].  .  .  . 

*  *  *  *  *  **  * 

The  trade  from  the  Grande  Portage  is,  in  some  particulars,  carried  on  in  a  dif- 
ferent manner  with  that  from  Montreal.     The  canoes  used  in  the  latter  transport  are 


«  Further  on  in  his  narrative,  Mackenzie  cites  examples  of  men  who  have  taken  seven  packages 
each,  and  carried  them  without  stopping  across  a  portage  one-half  league  in  length  [p.  Ixi.]— J.  M.  C. 

6  The  system  of  living  at  Grande  Portage  was  decidedly  feudal,  and  is  described  by  JNIackenzie 
as  follows:  "  The  proprietors,  clerks,  guides,  and  interpreters  mess  together,  to  the  number  of  some- 
times a  hundred,  at  several  tables,  in  one  large  hall,  the  provision  consisting  of  bread,  salt  pork, 
beef,  hams,  fish,  and  venison,  butter,  pease,  Indian  corn,  potatoes,  tea,  spirits,  wine,  etc.,  and  plenty 
of  milk,  for  which  purpose  several  milch  cows  are  constantly  kept.  The  mechanics  have  rations  of 
such  provision,  but  the  canoemen  both  from  the  North  and  Montreal  have  no  other  allowance  here, 
or  in  the  voyage,  than  Indian  corn  and  melted  fat ' '  [p.  xlvi].  The  Indian  corn  mentioned  is  hominy 
prepared  at  Detroit. — J.  M.  C. 


60  THE  VERMILION  IRON-BEARING  DISTRICT. 

uow  too  large  for  the  former,  and  some  of  about  half  the  size  are  procured  from  the 
natives,  and  are  navigated  by  four,  five,  or  six  men,  according  to  the  distance  which 
they  have  to  go.  The}'  carry  a  lading  of  about  thirty -live  packages,  on  an  average; 
of  these,  twenty-three  are  for  the  purpose  of  trade,  and  the  rest  are  employed  for  pro- 
visions, stores,  and  baggage.  In  each  of  these  canoes  are  a  foreman  and  steersman  — 
the  one  to  be  always  on  the  lookout,  and  direct  the  passage  of  the  vessel,  and  the 
other  to  attend  the  helm.  They  also  carry  her  whenever  that  office  is  necessary. 
The  foreman  has  the  command,  and  the  middle  men  obey  both;  the  latter  earn  only 
two-thirds  of  the  wages  which  are  paid  the  two  former.  Independent  of  these  a 
conductor  or  pilot  is  appointed  to  every  four  or  six  of  these  canoes,  whom  they  are 
all  obliged  to  obey,  and  is,  or  at  least  is  intended  to  be,  a  person  of  superior 
experience,  for  which  he  is  proportionabh'  paid. 

In  these  canoes,  thus  loaded,  they  embark  at  the  north  side  of  the  portage,  on 
the  river  Au  Tourt,"^*  which  is  ver}'  inconsiderable,  and,  after  about  2  miles  of  a 
westerlj'  course,  is  obstructed  by  the  Partridge  Portage,  600  paces  long.  In  the 
spring  this  makes  a  considerable  fall,  when  the  water  is  high,  over  a  perpendicular 
rock  of  120  feet.  From  thence  the  river  continues  to  be  shallow,  and  requires  great 
care  to  prevent  the  bottom  of  the  canoe  from  being  injured  bv  sharp  rocks,  for  a 
distance  of  oi  miles  to  the  prairie,  or  meadow,  when  half  the  lading  is  taken  out,  and 
carried  hy  part  of  the  crew,  while  two  of  them  are  conducting  the  canoe  among  the 
rocks  with  the  remainder  to  the  Carreboeuf  Portage,  3i  miles  more,  when  they 
unload  and  come  back  2  miles  and  embark  what  was  left  for  the  other  hands  to 
carrj",  which  they  also  land  with  the  former,  all  of  which  is  carried  680  paces,  and 
the  canoe  led  up  against  the  rapid.  From  hence  the  water  is  better  calculated  to 
carrj'  canoes,  and  leads  b}-  a  winding  course  to  the  north  of  west  3  miles  to  the 
Outard  Portage,*  over  which  the  canoe,  and  everything  m  her,  is  carried  for  2,400 
paces.  At  the  farther  end  is  a  very  high  hill  to  descend,  over  which  hangs  a  rock 
upwaid  of  700  feet  high.  Then  succeeds  the  Outard  Lake,''  about  6  miles  long,  lying 
in  a  northwest  course,  and  about  2  miles  wide  in  the  broadest  part.  After  passing  a 
very  small  rivulet  thej^  come  to  the  Elk  Portage,''  over  which  the  canoe  and  lading 
are  again  carried  1,120  paces,  when  they  enter  the  lake  of  the  same  name,  which  is  a 
handsome  piece  of  water,  running  northwest  about  -4  miles,  and  not  more  than  1^ 
miles  wide."  They  then  land  at  the  Portage  de  Cerise,"^  ovei  which,  and  in  the  face 
of  a  considerable  hill,  the  canoe  and  cargo  are  again  transported  for  1,050  paces. 
This  is  only  separated  from  the  second  Portage  de  Cerise  by  a  mud  pond''  (where 

*  Here  is  a  most  excellent  fishery  for  whitefish,  which  are  exquisite. 

"  According  to  Coues,  tliis  is  the  abbreviation  for  Tourtes.  This  was  even  then  known  as  Pigeon 
River.— J.  M.  C. 

''Known  now  as  Fowl  Portage. — J.  Jl.C. 

« Called  on  Thompson's  map,  1792,  Goose  Lake.     Now  called  South  Fowl  Lake. — J.  M.  C. 

''Called  now  and  also  on  Thompson's  map  Moose  Lake  and  Portage. — J.  M.  C. 

'Big  Cherry  Portage  now. — J.  M.  C. 

./■Mud  Portage.— J.  M.  C. 


RESUME  OF  LITERATURE.  61 

there  is  plentj^  of  water  lilies)  of  a  quarter  of  a  mile  in  leugth,  and  this  is  again 
separated  by  a  similar  pond  from  the  last  Portage  de  Cerise/'  which  is  410  paces. 
Here  the  same  operation  is  to  be  performed  for  380  paces.  Thej"  next  enter  on  the 
Mountain  Lake,  running  northwest  b}^  west,  6  miles  long  and  about  2  miles  in  its 
greatest  breadth.  In  the  center  of  this  lake  and  to  the  right  is  the  Old  Road,  by 
which  I  never  passed ;  but  an  adequate  notion  maj'  be  formed  of  it  from  the  road  1 
am  going  to  describe,  and  which  is  universallj^  preferred.  This  is,  first,  the  small 
new  portage''  over  which  everj'thing  is  carried  for  626  paces,  over  hills  aud  gullies. 
The  whole  is  then  embarked  on  a  narrow  line  of  water  '^  that  meander's  southwest 
about  2i  miles.  It  is  necessary  to  unload  here,  for  the  length  of  the  canoe,  and  then 
proceed  west  half  a  mile  to  the  new  Grande  Portage,  which  is  3,100  paces  in  length, 
and  over  very  rough  ground,  which  requires  the  utmost  exertions  of  the  men,  and 
frequently  lames  them;  from  hence  thej'  approach  the  Rose  Lake,  the  portage  of 
that  name  being  opposite  to  the  junction  of  the  road  from  the  Mountain  Lake.  They 
then  embark  on  the  Rose  Lake,  about  1  mile  from  the  east  end  of  it,  and  steer  west 
by  south  in  an  oblique  course  across  it,  2  miles;  then  west-northwest,  passing  the 
Petite  Perche  to  the  Marten  Portage,  3  miles.  In  this  part  of  the  lake  the  bottom  is 
mud  and  slime,  with  about  3  or  4  feet  of  water  over  it;  and  here  I  frequently  stuck 
a  canoe  pole  of  12  feet  long  without  meeting  anj'  other  obstruction  than  if  the  whole 
were  water.  It  has,  however,  a  peculiar  suction  or  attractive  power,  so  that  it  is 
difficult  to  paddle  a  canoe  over  it.''  There  is  a  small  space  along  the  south  shore 
where  the  water  is  deep,  and  this  efl'ect  is  not  felt.  In  proportion  to  the  distance 
from  this  part,  the  suction  becomes  more  powerful.  I  have,  indeed,  been  told  that 
loaded  canoes  have  been  in  danger  of  being  swallowed  up,  and  have  only  owed  their 
preservation  to  other  canoes,  which  were  lighter.  I  have,  myself,  found  it  difficult 
to  get  away  from  this  attractive  power,  with  6  men  and  great  exertion,  though  they 
did  not  appear  to  be  in  any  danger  of  sinking. 

Over  ao-ainst  this  is  a  verv  high,  rockv  ridge,  on  the  south  side,  called  Marten 
Portage,  which  is  but  20  paces  long,  and  separated  from  the  Perche  Portage,  which  is 
480  paces,  hj  a  mud  pond  covered  with  white  lilies.  From  hence  the  course  is  on 
the  lake  of  the  same  name,"  west-southwest  3  miles  to  the  height  of  land,  where  the 
waters  of  the  Dove  or  Pigeon  River  terminate,  and  which  is  one  of  the  sources  of 
the  great  St.  Laurence  in  this  direction.    Having  carried  the  canoe  and  lading  over  it, 

« Little  Cherry  Portage.— J.  M.  C. 

6  Watab  Portage  on  Minnesota  geological  survey  maps. — J.  M.  C. 

cThis  is  Eove  Lake.— J.  M.  C. 

''This  phenomenon  is  very  familiar  to  every  one  who  has  used  a  canoe.  After  paddling  over 
one  of  these  muddy  bottoms,  apparently  barely  making  the  canoe  move  with  the  greatest  exertion, 
there  is  a  remarkable  sense  of  relief  and  an  increase  in  the  rapidity  of  the  canoe's  motion  -when  there  ■ 
is  a  change  to  a  sandy  or  gravelly  bottom.  An  increase  in  depth  of  the  water  between  the  top  of  the 
mud  and  the  bottom  of  the  canoe  will  have  essentially  the  same  effect.  The  cause  of  this  is  that  the 
canoe  is  actually  floating  in  a  thin  mud,  and  the  friction  between  this  mud  and  the  canoe  is  \'ery  much 
greater  than  between  the  clear  water  and  the  canoe. — J.  M.  C. 

«Now  called  South  Lake.— J.  M.  C. 


62  THE  VERMILIOX  IRON-BEARING  DISTRICT. 

679  paces,  they  embark  on  the  lake  of  Hauteur  de  Terre,*  "  which  is  in  the  shape  of  an 
horseshoe.  It  is  entered  near  the  curve  and  left  at  the  extremity-  of  the  western 
limb,  through  a  very  shallow  channel,  where  the  canoe  passes  half  loaded  for  30 
paces  with  the  current,  which  leads  through  the  succeeding  lakes  and  rivers  and 
disembogues  itself  by  the  River  Nelson  into  Hudson's  Bay.  The  tirst  of  these  is  Lac 
de  Pierres  ii  Fusil,*  running  west-southwest,  7  miles  long  and  2  wide,  and  making  an 
angle  at  northwest  1  mile  more,  becomes  a  river  for  half  a  mile,  tumbling  over  a 
rock  and  forming  a  fall  and  portage,  called  the  Escalier,''  of  55  paces;  but  fi-om 
hence  it  is  neither  lake  or  rivei",  but  possesses  the  chai-acter  of  both,  and  ends 
between  large  i-ocks,  which  cause  a  current  or  rapid,  falling  into  a  lake  pond  for 
about  2i  miles,  west-northwest,  to  the  portage  of  the  Cheval  du  Bois.-'  Here  the 
canoe  and  contents  are  carried  380  paces  between  rocks;  and  within  a  quarter  of  a 
mile  is  the  Portage  des  Gros  Pins,'°  which  is  640  paces  over  a  high  ridge.  The 
opposite  side  of  it  is  washed  by  a  small  lake  3  miles  round;  and  the  course  is  through 
the  east  end  or  side  of  it,  three-quarters  of  a  mile  northeast,  where  there  is  a  rapid. 
An  irregular,  meandering  channel,  between  rocky  banks,  then  succeeds  for  7^  miles 
to  the  Maraboeuf  Lake,  which  extends  north  i  miles,  and  is  three-quarters  of  a  mile 
wide,  terminating  by  a  rapid  and  decharge  of  180  paces,  the  rock  of  Saginaga  being 
in  sight,  which  causes  a  fall  of  about  7  feet  and  a  portage  of  55  paces. 

Lake  Saginaga  takes  its  name  from  its  numerous  islands.  Its  greatest  length 
from  east  to  west  is  about  14  miles,  with  verj'  irregular  inlets;  is  nowhere  more 
than  3  miles  wide,  and  terminates  at  the  small  portage  of  La  Roche,-''  of  43  paces. 
From  thence  is  a  rocky,  stony  passage  of  1  mile  to  Prairie  Portage,  which  is  very 
improperly  named,  as  there  is  no  ground  about  it  that  answers  to  that  description, 
except  a  small  spot  at  the  embarking  place  at  the  west  end.  To  the  east  is  an  entire 
bog,  and  it  is  with  great  difficulty  that  the  lading  can  be  landed  upon  stages,  formed 
by  driving  piles  into  the  mud,  and  spreading  branches  of  trees  over  them.  The 
portage  rises  on  a  stony  ridge,  over  which  the  canoe  and  cargo  must  be  carried  for  611 
paces.  This  is  succeeded  bj-  an  embarkation  on  a  small  baj',^'  where  the  bottom  is 
the  same  as  has  been  described  in  the  west  end  of  Rose  Lake,  and  it  is  with  great 
difficulty  that  a  laden  canoe  is  worked  over  it,  but  it  does  not  comprehend  more  than 
a  distance  of  200  yards.  From  hence  the  progress  continues  thi-ough  irregular 
channels,  bounded  bv  rocks,  in  a  westerlv  course  for  about  5  miles  to  the  little 


*  The  route  which  we  have  been  traveling  hitherto  leads  along  the  high,  rocky  land  or  bank  of 
Lake  Superior  on  the  left.  The  face  of  the  country  offers  a  wild  scene  of  huge  hills  and  rocks, 
separated  by  stony  valleys,  lakes,  and  ponds.  Wherever  there  is  the  least  soil  it  is  well  covered  with 
trees. 

"  Now  called  North  Lake.— J.  M.  C. 

''GunflintLake.— .J.  M.  C. 

'Little  Rock  Portage.— J.  M.  C. 

''Wood  Horse  Portage. — J.  M.  C. 

<'  Pine  Portage. — J.  jM.  C. 

./"This  leads  from  Saganaga  into  Swamp  Lake. — J.  I\L  C. 

<J  At  east  end  of  Otter  Track  Lake.— J.  M.  C. 


RESUME  OF  LITERATURE.  63 

Portage  des  Couteaux,  of  165  paces,  and  the  Lac  des  Couteaux,  running  about 
southwest  by  west  12  miles,  and  from  a  quarter  to  2  miles  wide.  A  deep  bay  runs 
east  3  miles  from  the  west  end,  where  it  is  discharged  by  a  rapid  river,  and  after 
running  2  miles  west  it  again  becomes  still  water.  In  this  river  are  two  carrying 
places,  the  one  15  and  the  other  190  paces.  From  this  to  the  Portage  des  Carpes  is 
1  mile  northwest,  leaving  a  narrow  lake  on  the  east  that  runs  parallel  with  the  Lake 
des  Couteaux,  half  its  length,  where  there  is  a  carrj'ing  place  which  is  used  when 
the  water  in  the  xiver  last  liientioned  is  too  low.  The  Portage  des  Carpes  is  390 
paces,  from  whence  the  water  spreads  irregularlj^  between  rocks  5  miles  northwest 
and  southeast  to  the  portage  of  Lac  Bois  Blanc,  which  is  ISO  paces.  Then  follows 
the  lake  of  that  name,"  but  I  think  improperly  so  called,  as  the  natives  name  it  the 
Lake  Pascau  Minac  Sagaigan,  or  Drj'  Berries. 

Before  the  smallpox  ravaged  this  country  and  completed  what  the  Nodowasis 
[Sioux]  in  their  warfare  had  gone  so  far  to  accomplish,  the  destruction  of  its  inhab- 
itants, the  population  was  very  numerous;  this  was  also  a  favourite  part,  where 
they  made  their  canoes,  etc. ,  the  lake  abounding  in  fish,  the  countrjr  round  it  being 
plentifully  supplied  with  various  kinds  of  game,  and  the  rockj^  ridges,  that  form  the 
boundaries  of  the  water,  covered  with  a  variety  of  berries. 

When  the  French  were  in  possession  of  this  country  the}^  had  several  trading 
establishments  on  the  islands  and  banks  of  this  lake.  Since  that  period  the  few  people 
remaining,  who  were  of  the  Algonquin  Nation,  could  hardly  find  subsistence;  game 
having  become  so  scarce  that  they  depended  princi]3allj'  for  food  upon  fish  and  wild 
rice,  which  grows  spontaneously  in  these  parts. 

This  lake  is  irregular  in  its  form,  and  its  utmost  extent  from  east  to  west  is  15 
miles;  a  point  of  land  called  Point  au  Pin,  jutting  into  it,  divides  it  in  i^o  parts; 
it  then  makes  a  second  angle  at  the  west  end  to  the  lesser  Portage  de  Bois  Blanc,  .200 
paces  in  length. 

This  description  will  serve  to  give  the  reader  unacquainted  with  the 
area  some  knowledge  of  the  character  of  the  route  and  of  the  character  of 
the  voyagevu's  and  Indians.  Further  interesting  details  of  the  international 
canoe  route  and  the  methods  of  travel  can  be  obtained  from  Alexander 
Henry's  and  David  Thompson's  journals  for  the  years  1799-1814,  pp. 
xlvii-liii.'' 

"Commonly  called  at  present  Basswood  Lake. — J.  M.  C. 

''New  light  on  the  early  history  of  the  Greater  Northwest.  The  manuscript  journals  of  Alexander 
Henry,  fur  trader  of  the  Northwest  Company,  and  of  David  Thompson,  official  geographer  and  explorer 
of  the  same  company,  1799-1814.  Edited  by  Elliott  Coues.  New  York:  Francis  P.  Harper,  in  three 
volumes     1897. 


64  THE  VERMILION  IRON-BEARING  DISTRICT. 

GEOLOGICAL,  UTERATtiRE. 
1825. 

BiGSBT,  John  J.  Notes  on  the  geography  and  geology  of  Lake  Superior: 
Quarterl}^  Journal  of  Science,  Literature,  and  Arts,  Vol.  XVIII,  1825,  pp.  1-34,  222- 
269,  with  map. 

Bigsby,  in  1825,  describes  the  rocks  at  man}'  points  along  the  north 
shore  of  Lake  Superior.  He  also  follows  the  old  route  from  Lake  Superior 
to  the  Lake  of  the  Woods  for  430  miles,  and  observes  an  alternation  of  chlo- 
ritic  greenstone  and  amphibolitic  granite.  Near  and  on  the  Lake  of  the 
Woods  the  greenstone  passes  into  gneiss  and  mica-slate  which  is  peneti'ated 
bv  graphic  granite. 

This  statement  concerning  Bigsby's  observations  is  obtained  from  Bul- 
letin U.  S.  Geological  Survey  No.  86,  p.  51.  The  article  to  which  these 
statements  were  credited  not  having  been  found,  they  can  not  be  verified; 
and  whether  any  further  observations  were  made  by  Bigsbj-  in  the  area 
included  in  the  Vermilion  district  has  not  been  learned. 

1841. 

Houghton,  Douglass.  [Fourth]  Annual  Report  of  the  State  Geologist,  1841. 
State  of  Michigan.  House  of  Representatives,  No.  27.  Reprint  in  "Memoirs  of 
Douglass  Houghton,  First  State  Geologist  of  Michigan,"  bj'  Alvah  Bradish,  Detroit, 
1889;  302  pages. 

In  this  report  Dr.  Houghton  refers  incidentally  to  that  poition  of 
Minnesota  which  extends  along  the  international  boundarj-,  a  part  of  which 
is  included  in  this  monograph,  in  the  following-  words : 

The  hills  rise  in  broad  and  somewhat  knobby  steppes  or  plateaus,  to  heights 
varying  from  400  to  1,200  feet  above  the  lake,  and  the  summits  of  these  hills  are 
usually  not  farther  inland  than  from  10  to  20  miles.  The  rocks  of  the  hills  are  very 
frequently-  bare  over  considerable  areas,  and  the  vallej-s  containing  arable  soil  are 
few  and  very  narrow. 

The  route  of  the  fur  trade  to  the  northwest,  via  Rainy  lakes.  Lake  of  the  Woods, 
and  Lake  Winnepic,  was  formerly  wholly  carried  on  by  passing  over  these  hills 
from  a  point  a  few  miles  west  from  the  mouth  of  Pigeon  River.  The  trail  or 
portage  path  passes  over  a  low  jDortion  of  the  range,  and  tinally  falls  upon  Pigeon 
River,  which  is  ascended  to  its  source,  from  which,  l\v  a  series  of  portages,  the 
sources  of  the  streams  flowing  northwesterly  are  reached.  The  hilly  portion  of  the 
country,  though  of  exceeding  interest  in  a  geological  point  of  view,  is  the  most 
desolate  that  could  be  conceived. 


RESUME  OF  LITERATURE.  65 

1852. 

Owen,  David  Dale.  Report  of  a  geological  surve}'  of  Wisconsin,  Iowa,  and 
Minnesota;  and  incidentally  of  a  portion  of  Nebraska  Territory'.  1852.  (Dr.  J.  G. 
Norwood's  report,  pp.  213-ilS.) 

Ill  1848  Dr.  Norwood  ascended  St.  Louis  River,  and  crossing  the 
divide  descended  Vermilion  River  to  Vermilion  Lake.  This  portion  of  the 
river  is  now  known  as  Pike  River.  He  then  crossed  the  lake  and  went  on 
down  Vermihon  River  proper  to  Rainy  River.  His  observations  were  not 
numerous.  On  the  divide,  the  Missab^  Wachu  (Big  Man  Hills)  or  Giants 
range,  he  observed  syenitic  granite  associated  with  gneiss.  As  Vermilion 
Lake  was  approached,  outcrops  of  slaty  hornblende  rock,  micaceous  clay 
slate,  and  siliceous  slate  appeared  in  the  banks  and  bed  of  the  river, 
forming  riffles  and  falls.  On  the  south  side  of  Vermilion  Lake  talcose  and 
mica-slate  and  micaceous  schists  were  exposed.  On  the  north  side  of  the 
lake,  at  the  outlet,  and  continuing  on  along  tlie  outlet,  mica-slate  and 
granite  were  found.  He  notes  that  the  general  trend  of  the  ridges  is  east- 
northeast  and  west-southwest  (p.  313).  Structurally,  he  considers  the 
northeast  part  of  Minnesota  (p.  333)  to  consist  of  northeast-southwest 
alternating  anticlinal  and  synclinal  folds,  rivers  sometimes  occupying  the 
synclinal  valleys.  A  range  of  greenstone,  beginning  at  the  great  bend  of 
St.  Louis  River  and  running  northeast  (N.  30°  E.),  forms  a  true  anticlinal 
axis,  the  line  of  elevation  crossing  the  boundary  line  between  the  sources 
of  Arrow  Lake  and  Mountain  Lake  (p.  3361." 

In  1849  Dr.  Norwood  followed  the  international  boundary  from  Pigeon 
River  to  Saganaga  Lake.  He  mentions  the  occurrence  of  siliceous  slate 
and  hornblende  and  ferruginous  rocks  on  Gunflint  Lake,  and  while  the 
statement  is  not  perfectly  clear,  he  seems  to  have  the  idea  that  these  have 
been  disturbed  by  the  intrusion  of  the  granite,  which  he  observed  exposed 
from  Gunflint  Lake  to  Saganaga  Lake.  This  granite  forms  a  range  which, 
if  continued  on  the  southwest,  he  states  (p.  417)  would  pass  in  the  line  of 
the  Missabe  Wachu  (Giants  range)  and  Pokegama  Falls  on  the  Mississippi. 
He  thus  implies  the  con-elation  of  the  granite  of  Saganaga  Lake  with  that 
of  the  Mesabi  range,  a  position  taken  by  some  of  the  later  geologists,  as 
will  be  seen  below.    He  commends  the  northwest  shore  of  Lake  Superior  as 

a  Norwood  erred  in  making  this  statement,  as  this  axis — the  Giants  range — crosses  the  interna- 
tional boundary  between  Gunflint  and  Saganaga  lalies. 

MON  XLV — 03 5 


QQ  THE  VERMILION  IRON-BEARING  DISTRICT. 

perhaps  the  best  extinct  volcanic  region  in  the  ^Yorld  in  wliich  to  study 
igneous  intrusion.  It  is  to  be  regretted  that  Owen  did  not  closely  follow 
Norwood's  •  notes  in  compiling  his  general  map,  as  the  map  would  then 
show  far  more  accurately  than  it  does  the  distribution  of  the  rocks  of  the 
Vermillion  district  as  disclosed  by  Norword's  reconnaissance  survey. 

Sections  1  and  2  on  PL  2  N  of  Owen's  report  have  no  legend  other 
than  that  giving  a  general  location,  and  consequently  one  can  not  be  abso- 
luteh-  sure  of  the  rock  represented.  It  is  clear,  however,  that  Norwood  had 
the  idea  that  the  metamorphic  rocks  in  both  the  Vermillion  and  the  Saganaga 
area  are  cut  by  the  granite  of  the  Mesabi  range  and  Saganaga  Lake  and 
folded  in  between  the  granite  I'idges. 

1S61. 

Anderson,  C.  L..  and  Clark,  Thomas.  Report  on  geolog-y:  Document  No.  12, 
Minnesota  legislature.     St.  Paul,  1S61;  26  pages. 

There  was  reprinted  in  1 860,  by  order  of  the  Minnesota  senate,  portions 
of  the  publications  of  the  geological  surveys  of  Wisconsin  for  1854  and  1868, 
which,  owing  to  the  juxtaposition  of  the  States,  it  was  thought  would  equally 
apply  to  Minnesota.  Mr.  Thomas  Clark,  of  Lake  City,  was  chairman  of  the 
committee  having  in  charge  the  publication  of  this  first  geologr>.al  report  of 
the  State  of  Minnesota.  The  same  year  that  this  report  was  published  a 
commission  was  appointed,  consisting  of  C.  L.  Anderson  and  Thomas  Clark, 
above  referred  to,  to  report  on  the  geology  of  the  State  and  on  a  plan  for  a 
geological  survey  of  the  State.  This  report  was  sent  to  the  legislature  by 
the  governor,  who,  however,  declared  himself  not  ready  to  advise  the  com- 
mencement of  a  geological  survey. 

In  the  report  the  commissioners  call  attention  to  some  of  the  general 
geologic  features  of  the  State.  The  only  points  of  interest  in  connection 
with  the  present  study  of  the  literature  of  the  region  is  the  recognition 
of  the  existence  of  granite  and  metamorphic  rocks,  forming  northeast- 
southwest  trending  ranges  in  the  northeastern  part  of  the  State,  and  the 
insistence  on  the  investigation  of  this  area,  with  the  view  to  determining  the 
existence  of  metalliferous  deposits  in  the  rocks  in  this  region. 

1S65. 
Hanchett,  a.  H..  and  Clark,  T.      Report  of  the  State  geologist,  Aug.  H. 
Hanohett,  ^I.  D.,  together  with  the  physical  geography,  meteorology,  and  liotany 
of  the  northeastern  district  of  Minnesota,   by  Thomas  Clark,  assistant  geologist. 
St.  Paul,  1865;  82  pages. 


RESUME  OF  LITERATURE.  67 

The  agitation  of  the  question  of  the  organization  of  a  geological  survey 
of  Minnesota  evidently  had  some  effect,  as  is  shown  in  the  publication  of 
this  report.  Among  other  things,  the  State  geologist  reports  that  "speci- 
mens of  hematitic  specular  iron  ore  were  obtained  from  a  heavy  deposit  said 
to  lay  between  a  lake  forming  the  affluence  of  the  Upper  Embarrass  River 
and  Vei'million  Lake.  The  precise  percentage  of  commercially  pure  iron 
contained  in  this  ore  has  not  been  ascertained"  (p.  6). 

1866. 

Eames,  Henry  H.  Report  of  the  State  geologist  on  the  metalliferous  region 
bordering  on  Lake  Superior.     St.  Paul,  1S66;  23  pages. 

Eames  gives  merely  a  brief  description  of  the  then  known  metalliferous 
rocks  of  northeastern  Minnesota.  He  mentions  the  occurrence  in  the  Ver- 
million district  of  the  siliceous  and  talcose  slates  (p.  10)  of  Vermillion  Lake, 
in  the  last  of  which  are  found  auriferous  and  argentiferous  quartz  veins  and 
the  hematite  iron  ore  (p.  1 1)  of  Vermillion  Lake,  which  is  associated  with 
quartzose  jasperoids  and  serpentine  rocks. 

Eames,  Henrt  H.  Geological  reconnoissance  of  the  northern,  middle,  and 
other  counties  of  Minnesota.     St.  Paul,  1866 ;  58  pages. 

This  contains  the  results  of  a  geologic  reconnaissance  of  the  State.  It 
was  found  that  the  granite  uplifts  have  their  greatest  development  in  the 
northeastern  part  of  the  State.  They  trend  northeast,  and  reach  their 
greatest  altitude  at  or  near  the  Missabe  heights.  The  most  prevalent  rocks 
found  in  the  northern  part  of  the  State  are  granite,  porphyry,  hornblendic 
slates,  siliceous  slates,  trap,  greenstone,  talcose  slate,  primitive  schistose 
rock,  gneiss,  and  Potsdam  sandstone.  Li  the  region  occupied  by  these 
rocks  are  found  immense  bodies  of  magnetitic  and  hematitic  iron  ore.  In 
the  talcose  slates  and  primitive  schistose  rock  are  veins  of  quartz,  carrying 
auriferous  and  argentiferous  sulphides  of  iron  and  copper 

In  a  report  by  Richard  M.  Eames,  assistant,  a  number  of  details  of  the 
Vermilion  Lake  district,  chiefly  concerning  veins,  are  given,  and  a  geologic 
map  showing  the  outline  of  the  formation  surrounding  Vermilion  Lake  is 
said  to  have  been  prepared  and  handed  in,  but  does  not  accompany  the 
published  report. 


68  THE  VERMILION  IRON-BEARING  DISTRICT. 

Whittlesei,  Col.  Charles.  Geology  and  minerals.  A  report  of  explora- 
tions in  the  mineral  regions  of  Minnesota  during-  the  years  lS-i8,  1S59.  and  lS6i. 
Cleveland,  1S66;  5-i  pages. 

According  to  Colonel  Whittlesey  the  Minnesota  shore  of  Lake  Superior 
structurally  represents  the  northern  edge  of  a  synchue,  in  the  basin  of 
which  Lake  Superior  lies,  and  we  consequently  get  essentially  the  same 
succession  of  rocks  that  is  found  on  the  southern  shore  of  Lake  Superior 
in  j\Iichig-an  and  Wisconsin — a  belt  of  alternating  sandstone  beds  and  trap 
flows,  succeeded  inland  by  a  belt  of  trap.  These  two  constitute  the  series 
which  is  known  as  the  Keweenawan,  Back  of  the  trap  he  finds  a  belt  of 
hornblende  rocks,  or  hornblende-slates,  as  they  are  called.  This  is  the 
Animikie  series  of  the  Minnesota  geologists. 

Behind  the  hornblende  system  is  the  imperfectly  defined  region  of  the  granite, 
syenite,  mica  slate,  siliceous,  and  talcose  rocks,  extending  to  and  across  the  national 
bouudarj'.  The  Mesabi  range  occupies  the  watershed  between  the  waters  of  Lake 
Superior  and  those  of  Hudson's  Baj'.  In  many  cases  the  sj^enite  and  granite  appears 
to  be  more  recent  than  the  metamorphic  slates,  having  all  the  appearance  of 
intrusive  rocks  [p.  7]. 

That  part  of  the  Vermilion  Lake  region,  including  the  rocks  just 
mentioned — the  mica-slate,  siliceous  and  talcose  slates — which  lies  to  the 
north  of  the  Mesabi  range  is  described  in  some  detail.  Leaving  the  syenitic 
Mesabi  range  he  proceeds  over  the  portage  trail  to  Vermilion  River  (now 
known  as  Pike  River)  and  passes  in  the  order  given  over  syenite,  "gray, 
compact  quartz,"  mica-slate,  and  distinctly  layered  novaculitic  quartzite. 
Just  before  entering  Vermilion  Lake  fine-grained  micaceous  rocks  are 
ex^aosed,  and  these  continue  along  the  western  shore,  becoming  more 
talcose  and  slaty  as  the  explorer  goes  north.  The  slaty  laminje  strike 
northeast  by  east,  and  dip  75°-80°  northwest.  These  slates  are  also  very 
much  jointed,  causing  the  formation  of  rhombohedral  blocks.  On  the  north 
shore  of  Vermilion  Lake,  Colonel  Whittlesey  saw  and  recognized  clearly 
the  relations  of  the  granite  to  the  sedimentaries,  mica  and  talcose  slate,  as 
he  calls  the  rocks.  He  says,  "The  granite  appears  as  a  protrusion  in  the 
slates,  and  is  therefore  more  recent"  (p.  45). 


RESUME  OF  LITERATURE.  69 

1871. 

Kloos,  J.  H.  Article  in  The  Minnesota  Teacher,  referred  to  by  N.  H. 
Winchell. 

Wiuchell "  says  that  Kloos  suggests  the  possibility  of  the  hematitic 
and  magnetitic  iron  ore  at  Vermilion  Lake  being  in  the  lowest  member  of 
the  Huronian. 

Kloos,  J.  H.  Geologische  Notizen  aus  Minnesota:  Zeitschr.  deutsch.  geol. 
Ges.,  Vol.  XXIII,  1871,  pp.  417-W8.* 

In  this  article  (p.  199)  Kloos  states  that  he  has  become  acquainted 
with  gneisses  and  finely  crystalline  clay  slates  from  Vermilion  Lake,  which 
appear  to  him  to  belong  to  the  Laurentian.  A  small  rush  to  this  area  was 
caused  by  the  discover}^  of  gold-bearing  pyi-itiferous  quartz  veins,  cutting 
the  metamorphic  schists,  but  did  not  result  in  the  opening  up  of  productive 
mines.  He  farther  on  mentions  having  heard  favorable  reports  concerning 
the  iron  ores  of  Vermilion  Lake. 

1873. 

Bell,  Robert.  Report  on  the  country  between  Lake  Superior  and  Lake 
Winnipeg:  Geol.  Survey  of  Canada;  Report  of  Progress  for  1872  and  1873, 
1873,  pp.  87-111. 

This  contains  a  report  (pp.  92-94)  of  a  reconnaissance  along  a  j^art  of 
the  international  boundary  from  Guuflint  Lake  to  Whitewood "  (Basswood) 
Lake,  which  is  included  in  the  Vermilion  district.  He  observed  sedimenta- 
ries,  slates,  and  dolomite  associated  with  trap  on  Gunflint  Lake.  These  he 
correlated  with  the  Lake  Superior  coj)per-bearing  rocks.  They  are  suc- 
ceeded to  the  northwest  by  the  Laurentian  granite  of  Seiganagah  (Saganaga) 
Lake.  West  of  this  Laurentian  area  the  route  crosses  from  Poplar  Lake 
(Swamp  Lake  of  the  boundary  commission  map)  to  Whitewood  (Basswood) 
Lake,  a  belt  of  Huronian  black  to  green  and  gray  schists,  slates,  and 
quartzites,  cut  by  dikes  of  trap.  The  sediments  strike  S.  15°-30°  W.,  and 
dip  about  80°  W.  The  only  rocks  observed  around  the  shores  of  White- 
wood  Lake  are  fine-grained  gi'ay  to  reddish-gray  syenite. 

«N.  H.  Winchell:  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Kept.,  Vol.  I,  1884,  p.  103. 

t>  A  translation  of  this  article  by  X.  H.  Winchell  is  published  in  Geol.  and  Nat.  Hist-  Survey  of 
Minnesota,  Tenth  Ann.  Rept.,  1SS2,  pp.  175-200. 

<■  The  usual  name  given  to  this  lake  is  Basswood.  No  basswood  was  seen  by  Bell  about  the  lake, 
however,  and  my  own  observation  showed  me  that  it  is  certainly  not  present  in  great  quantity  there. 
He  states  that  "  the  lake  is  said  to  derive  its  name  Lac  de  Bois  Blanc,  or  AVhitewood  Lake,  from  the 
whitewood,  or  balm  of  Gilead,  a  kind  of  poplar"  (p.  94). 


7(»  THE  VERMILION  IKON-BEARING  DISTRICT. 

WiNCHELL,  N.  H.  First  Ann.  Kept.  Geol.  and  Nat.  Hist.  Survey  ^Nlinn..  pp. 
129.     Second  edition,  1884. 

The  general  distribution  of  the  nonfo.ssil-bearing  rocks  of  Minnesota, 
referred  to  as  "granitic  and  metamorphic  rocks"  (p.  64),  is  given  in  this, 
tlie  first  annual  report  of  the  second  Minnesota  survey.  It  is  stated  that 
they  occup)-  a  great  portion  of  the  northern  part  of  the  State.  These 
rocks  are  regarded  as  Laurentian  and  Huronian.  At  the  time  of  this 
publication  the  great  deposits  of  iron  ore,  which  are  now  of"  such  national 
as  well  as  local  importance,  were  but  little  known,  as  appears  from  the 
indefinite  statement  that  "iron  ore  in  unlimited  quantities  is  said  to  exist 
in  the  dividing  ridge  between  Lake  Superior  and  Vermilion  Lake"  (p.  67). 

1S76. 

Whittlesey,  Col.  Charles.  Physical  geology  of  Lake  Superior:  Proc.  Am. 
Assoc.  Adv.  Sci.,  Twenty-fourth  Meeting,  1875,  Part  2,  pp.  60-72,  with  map, 

Whittlesey  finds  nowhere  on  the  American  side  of  the  boundary,  except 
at  Vermilion  Lake,  rocks  which  are  like  the  Laurentian  of  Canada.  The 
great  masses  of  granite  and  syenite,  around  which  the  Huronian  is  formed, 
do  not  resemble  the  Laurentian  of  the  Canadian  geologists.  Between  the 
Canadian  and  American  Huronian  there  is  a  very  close  resemblance.  The 
conclusion  of  Foster  and  Whitney  that  the  traps  of  Lake  Superior  are  of 
Potsdam  age  is  adopted. 

1877. 

Streng,  a.,  and  Kloos,  J.  H.  Ueber  die  krystallinischen  Gesteine  von  Minne- 
sota in  Nord-Ame'rika:  Neues  Jahrbuch  fiir  Mineralogie,  Geologic,  und  Paleonto- 
logie,  1877,  pp.  31-56,  113-138,  225-242;  translation,  by  N.  H.  Winchell,  in  the 
Eleventh  Ann.  Kept.  Geol.  and  Nat.  Hist.  Survey  Minn.,  for  1882,  pp.  30-85. 

■  The  authors  state  that  "the  St.  Louis  River  rises  in  northeastern  Minne- 
sota south  of  Vermilion  Lake,  in  a  region  of  granite,  gneiss,  and  crystalline 
slates,  which  form  a  branch  from  the  Laurentian  formation  as  it  is  displayed 
in  the  region  north  of  Lake  Superior"  (p.  36). 

1879. 

Winchell,  N.  H.  Seventh  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  JNIinn..  for 
1S78,  1S7'J,  pp.  9-25. 

In  this  we  find  a  report  by  C.  W.  Hall  on  a  reconnaissance  in  the 
northeastern  part  of  Minnesota.     In  the  summary  of  geologic  results  (p.  10) 


EESUME  OF  LITERATURE.  71 

the  trend  of  the  formations  is  stated  to  be  more  nearly  east  and  west  than 
it  had  previously  been  supposed  to  be.     Hence  the  formations — 

cross  the  coast  line  at  an  acute  ano-le,  the  later  formations  being-  toward  the  south. 
and  the  older  along  the  international  boundary  line.  .  .  .  The  formations  that 
compose  the  coast  line"  .  .  .  seem  to  be  something  a^i  follows,  in  descending  order: 
(1)  Metamorphic  shales,  sandstones,  and  quartzite.  These  are  cut  by  dikes,  and  are 
interbedded  with  igneous  rock.  ...  (2)  Ferruginous  and  aluminous  sandstones. 
These  seem  to  be  metamorphosed  into  a  iii'm  basaltiform  red  rock,  as  seen  in  the 
Palisades  and  at  other  points.  ...  (3)  A  quartzose  conglomerate,  seen  at  the  Great 
Palisades  and  Portage  Ba}'  Island — probably  more  properly  a  part  of  No.  2.  (4)  The 
quartzites  and  slates  of  Grand  Portage  Bay.  ...  (5)  The  jasper,  flint,  and  iron- 
bearing  belt  of  Gunflint  Lake  and  Vermilion  Lake,  and  of  the  INIesabi  range.  (6)  The 
slates  and  schists  which  the  Canadian  geologists  particularly'  designate  Huronian. 
(7)  The  syenites,  granites,  and  other  rocks  that  have  been  classed  as  Laurentian.  (8) 
The  igneous  rocks,  known  as  the  Cupriferous  series  [p.  10]. 

Of  these,  the  onl}'  ones  with  which  we  are  much  concerned  in  the 
Vermilion  district  are  Nos.  4,  5,  6,  7,  and  8.  It  is  therefore  of  interest  to 
learn  the  relations  of  these  to  one  another,  as  given  in  the  report. 

Nos.  i,  5,  6,  and  7  are  probabl}-  conformably  arranged  in  succession;  at  least 
thej^  have  been  so  seen  at  places.  Nos.  i  and  5  are  closely  associated,  and  perhaps 
the  latter  is  but  a  local  phase  of  the  former,  while  Nos.  6  and  7  are  as  closeh^  related, 
being  conformably  interbedded  and  stratified.  No.  5  is  conformable  with  No.  6  in 
the  iron  district  along  the  southeastern  side  of  Vermilion  Lake  [p.  11]. 

1S81. 

WiNCHELL,  N.  H.  Ninth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minn.,  for 
1880,  1881. 

That  portion  of  this  report  containing  a  mention  of  the  northeastern 
part  of  the  State  is  apparently  merely  the  published  field  notes  of  the  State 
geologist.  No  attempt  is  made  to  reduce  these  notes  to  an  orderl}^  discus- 
sion of  the  general  geology,  and  the  reader  must  glean  such  general  facts 
as  he  can  find  among  the  man}^  details  given.  Thus,  apro^jos  of  a  specimen 
taken  from  one  of  the  beds,  we  learn  that  a  greenish,  schistose,  porpln-ritic 
rock,  cut  by  veins  of  milk}-  quartz,  is  found  in  nearly  a  vertical  attitude 
on  Gunflint  Lake.  This  is  supposed  to  be  the  Canadian  Huronian,  and 
underlies    the    quartzite    and    Gunflint    beds,    apparently   unconformably 

«  The  northeast-southwest  trending  coast  line  of  Lake  Supei'ior  is  here  meant. 


72  THE  VERMILION  IRON-BEARING  DISTRICT. 

(p.  81).     The  Knife  Lake  chloritic  or  serpentinous  quartzite  is  regarded  as 
Huroniau  (p.  86). 

On  the  south  side  of  Vermihon  Lake  are  beds  of  jasper  and  iron 
hematite  whi(^h  are  regarded  as  the  equivalent  of  the  Gunflint  beds.  These 
are  conformable  with  the  magnesian  schists  and  slates  which  are  in  a 
vertical  attitude.  They  pass  down  into  the  schists,  and  in  places  the 
schists  and  schistose  structure  penetrate  the  jasper  and  iron  (pp.  103-4). 

WiNCHELL,  N.  H.  The  Cupriferous  series  in  Minnesota:  Proc.  Am.  Assoc. 
Adv.  Sci.,  Twenty-ninth  Meeting,  1880,  pp.  •122-425;  Ninth  Ann.  Rept.  Geol.  and 
Nat.  Hist.  Survey  Minn.,  for  1880,  1881,  pp.  385-387. 

In  this  description  of  the  Copper-bearing  series  the  impression  is  made 
that  the  series  gradually  becomes  more  and  more  changed  and  crystalline 
as  one  goes  away  from  the  shore  of  Lake  Superior.  The  following 
relations  are  mentioned: 

The  tilted  red  shales,  conglomerates,  and  sandstones  at  Fond  du  Lac  .... 
are  the  same  as  those  associated  with  the  igneous  rock  all  along  the  shore.  They 
lie  there  on  a  white  quartz  pebbly  conglomerate  of  a  few  feet  in  thickness,  which 
lies  unconformabl}^  on  the  roofing  slates  of  the  Huronian,  the  same  foi-mation 
which  succeeds  to  the  red-rock  formation  ....  at  Ogishke  Muncie  and  Knife 
lakes,  northwest  of  Grand  Marais  [p.  387]. 

1SS2. 

WiNCHELL,  N.  H.  Preliminary  list  of  rocks:  Tenth  Ann.  Rept.  Geol.  and 
Nat.  Hist.  Survey  Minn.,  for  1881,  18S2,  pp.  9-122. 

In  this  rejjort  the  publication  of  Winchell's  field  notes  is  continued. 
From  them  we  learn  that  the  iiint  and  jasper  formations  of  Gunflint  Lake 
appear  to  be  in  apparent  unconformit}^  with  the  underlying  slates  and 
syenites  (p.  88).     On  Ogishkie  Muncie  Lake  is  found  a  great  conglomerate. 

The  conglomerate  .  .  .  contains  large  rounded  pieces  of  the  "Saganaga 
granite,"  which  proves  the  greater  age  of  that  granite  and  the  unconformability 
of  this  slaty  conglomerate.   ...     The  conglomerate  also  here  contains  red  jasper 

[p.  ys]. 

The  descending  succession  in  northeastern  Minnesota  is  given  as 
follows: 

(1)  The  nearly  horizontal  quartzites  and  slate.  ...  (2)  The  coarse  grit 
or  fine  conglomerate.  (3)  The  jaspery  and  calcareous  .  .  .  Gunflint  bods.  (4) 
Gray   marble.     (5)  The   tilted,    slat}'   conglomerate,    and    the    great   conglomerate 


RESUME  OF  LITERATURE.  73 

about  Ogisiikie  Muncie  Lake  [bearing  Saganaga  granite  bowlders].  (6)  The  auiphi- 
bolite  and  chloritic  slates.  (7)  Mica  schists  and  alternations  of  mica  schists  and 
syenite.     (8)  The  syenites  and  granites  of  Saganaga  and  Gull  lakes  [p.  94]. 

Whether  the  great  quartzite  and  slate  formation  (No.  1  above)  is  the 
same  as  the  highly  tilted  slate  and  quartzite  formation  which  passes  into 
the  great  conglomerate  (No.  5  above)  of  Ogishkie  Muncie  Lake  is  an  open 
question,  although  there  are  several  things  which  indicate  that  they  are 
the  same.  They  have  been  treated  throughout,  however,  as  different 
terranes.  The  following'  are  given  as  the  considerations  which  appear  to 
support  their  equivalency : 

(a)  Where  the  horizontal  slates  approach  the  syenites  at  the  east  end  of  Gunfiint 
Lake  there  is  nothing  to  be  seen  of  any  beds  representing  the  tilted  slates.  The 
syenites  and  their  associated  schists  come  on  at  once,  {h)  Where  the  tilted  slates 
and  the  conglomerates  associated  with  them  are  traceable  from  the  syenite  upward 
to  the  gabbro,  as  south  of  Ogishkie  Muncie  Lake,  there  is  nothing  to  be  seen  of  any 
beds  like  the  horizontal  black  slates  of  No.  1.  (e)  The  "  Gunfiint  beds "  appear  to 
belong  to  the  horizontal  slates  of  the  international  boundar}^  at  Gunfiint  Lake,  but 
their  supposed  equivalents  at  Ogishkie  Muncie  Lake  belong  to  schistose  and  tilted 
slates  and  conglomerate,  {d)  Although  the  horizontal  slates  and  quartzites  of  the 
international  boundary  strike  west  and  southwest  across  the  State,  forming  one  of 
the  most  important  topographical  features  of  the  northern  part  of  the  State,  and  can 
be  followed  for  many  miles  as  such,  yet  they  are  lost  entireh^  in  the  region  of  the 
upper  St.  Louis,  and  the  tilted  slates  are  the  only  ones  seen  where  that  river  cuts 
the  rock  at  Knife  Falls  and  below,  (e)  The  great  gabbro  belt  which  surmounts  the 
horizontal  slates  along  the  international  boundar3^  and  prevails  to  the  east  and  south 
of  their  line  of  strike,  is  seen  to  pass  to  the  west  of  Lake  Superior  at  Duluth,  and 
to  disapjjear  from  sight  suddenl}'  between  Duluth  and  Fond  du  Lac  as  if  its  con- 
tinuance depended  on  the  maintenance  of  the  horizontal  formation  with  which  it  is 
associated.  (  f )  Where  the  Guuflint  beds  become  a  jaspery  hematite,  as  south  and 
east  of  Vermilion  Lake,  the  structure  of  the  tilted  slates  passes  into  the  iron  ore  as 
if  of  the  same  formation  [p.  95]. 

1883. 

Irving,  R.  D.  The  copper-bearing  rocks  of  Lake  Superior. :  Mon.  U.  S.  Geol. 
Survey  Vol.  V,  1883,  464  pp. ,  15  1. ,  29  pi.  and  maps.  See  also  Third  Ann.  Rept.  U.  S. 
Geol.  Survey,  1883,  pp.  89-188,  15  pi.  and  maps;  Science,  Vol.  I,  1SS3,  pp.  140,  359, 
and  422;  and  Am.  Jour.  Sci.,  3d  ser.,  Vol.  XXVIIl,  1884,  p.  462,  Vol.  XXIX,  1885, 
pp.  67-68,  258-259,  339-340. 

In  the  above  Irving  gives  a  detailed  account  of  the  copper-bearing 
rocks  of  Lake  Superior,  and  also  discusses  the  relations  of  this  series  to  the 


74  THE  VERMILION  IKON-BEARING  DISTRICT. 

rocks  (if  otlier  ag-es  with  wliicli  it  is  associated,  and  gives  many  details 
conceniiiig-  the  associated  rocks.  The  Copper-bearing  series  does  uot 
include  tlie  so-called  Lower  group  of  Logan,  the  Animikie  group  of  Hunt, 
and  also  the  horizontal  sandstones  known  as  tlie  Eastern  and  Western 
sandstones;  although  it  includes  the  dolomitic  sandstones,  -with  accompany- 
ing crystalline  rocks  occurring  between  Black  and  Thunder  bays,  in  the 
valleys  of  the  Black  Sturgeon  and  Nipigon  rivers,  and  about  Lake  Nipigon. 
The  Keweenaw  or  Copper-bearing  series  then  includes  the  succession  of 
interbedded  traps,  amygdaloids,  felsitic  porphyries,  porphyry-conglomerates, 
sandstones,  and  the  conformable  overlying  sand.stone  typicalh*  developed 
in  the  region  of  Keweenaw  Point  and  Portage  Lake.  These  rocks  have 
their  most  widespread  extent  about  the  western  half  of  Lake  Superior, 
but  also  occur  in  the  eastern  part  of  the  lake.  Their  entire  geographic 
extent  in  the  immediate  basin  of  Lake  Superior  is  about  41,000  square 
miles. 

The  Animikie  series  in  the  Thunder  Bay  district  is  of  great  thickness, 
probably  upward  of  10,000  feet,  comprising  quartzites,  quai-tz-slates,  clay 
slates,  magnetitic  quartzites,  sandstones,  thin  limestone  beds,  and  beds  of 
chert}'  and  jaspery  material.  With  these  are  associated  in  great  volume,  in 
both  interbedded  and  intersecting  masses,  coarse  gabbro  and  fine-grained 
diabase,  like  those  well  known  in  the  Keweenaw  series.  A  broad  examination 
of  the  region  shows  that  there  is  little  ground  for  the  belief  in  one  crowning 
overflow.  The  Animikie  series  is  lithologically  like  the  Penokee  series  in 
Wisconsin;  both  series  bear  the  same  relations  to  the  newer  Keweenawan 
rocks  and  the  older  gneisses,  and  the  two  groups  are  regarded  as  the  same. 

The  iinimikie  rocks  have  been  traced  by  Bell  and  also  by  N.  H. 
Winchell  as  far  west  as  Gunflint  Lake,  and  are  the  equivalent  of,  if  not 
actually  continuous  with,  the  Mesabi  iron  range  running  to  Pokegama 
Falls  and  the  slates  of  St.  Louis  Rivei',  although  the  latter  are  aftected  b}- 
slaty  cleavage. 

The  iron-bearing  schists  of  Vermilion  Lake  are  so  like  the  Huronian 
that  they  are  regarded  as  a  folded  continuation  of  the  Animikie  beds, 
and  a  generalized  section  showing  the  supposed  original  conneotiou  of 
the  Animikie  group  and  the  Yermihon  Lake  iron-bearing  schists  ovi-r  tlie 
c'ranite  of  the  Mesabi  rany-e  is  introduced. 

That   the  Animikie    Muronian   is   beneath  the   Keweenawan  rocks  is 


RfiSUMfi  OF  LITERATURE.  75 

shown  by  the  fact  that  the  Keweenawan  beds  along  the  Minnesota  coast 
are  passed  in  descending  order  until  the  Animikie  slates  are  reached  at 
Grand  Portage  Bay,  but  there  is  not  a  direct  downward  continuation  of  the 
Keweenawan  into  the  Animikie,  for  between  the  two  there  has  been  an 
intervennig  period  of  erosion.  This  is  shown  by  the  fact  that  at  Grrand 
Portage  Bay,  where  the  two  formations  come  together,  the  iinderlying  slates 
suddenly  rise  entirely  across  tlie  horizon  of  600  or  700  feet  of  the  Kewee- 
nawan sandstone.  Also  in  northeastern  Minnesota  and  in  the  Penokee 
district  the  overlying  Keweenawan  is  now  in  contact  with  one  member  of 
the  underlying  series  and  now  -with  another.  Further,  in  the  Keweenawan 
sandstones  of  Thunder  Bay  are  found  chert  and  jasper  pebbles  from  the 
Animikie,  while  in  the  Wisconsin  Keweenawan  are  quartzite  pebbles 
ai^parently  from  the  underlying  Huronian. 

18S4. 

Chester,  A.  H.  The  iron  region  of  northern  Minnesota:  Eleventh  Ann.  Rept. 
Geo],  and  Nat.  Hist.  Survey  Minn.,  for  1882,  1884,  pp.  15-1-167. 

This  report  gives  in  detail  the  result  of  two  expeditions  sent  out, 
the  one  in  1878,  the  other  in  1880,  by  private  parties  for  the  purpose  of 
exploring  the  reported  iron-ore  deposits  in  the  Mesabi  iron  range  and  on 
Vermilion  Lake.  The  earlier  of  these  two  expeditions  paid  little  attention 
to  the  Vermilion  Lake  deposits,  and  the  following  facts  were  obtained 
chiefly  as  the  result  of  the  expedition  in  1880. 

The  prevailing  rocks  in  the  Vermilion  Lake  iron  district  are  the  slates 
and  schists  and  mica-schists  and  quartzite  found  in  other  Hin-onian  areas  in 
connection  with  iron-ore  beds.  The  belt  of  iron  ore  is  well  defined.  The 
ore  is  found  in  connection  with  jasper  and  quartzite,  and  in  many  cases 
with  well-defined  walls  of  slate.  The  ore  deposits  are  intimately  bedded 
with  the  rocks  of  the  country,  slates,  schists,  and  mica-schists  and  quartzite, 
and  stand  nearly  vertical,  with  perhaps  a  slight  inclination  to  the  south, 
and  trend  generally  east  and  west,  though  this  varies  from  place  to  place. 
The  strata  are  much  folded  and  contorted  (p.  161). 

Exploration  developed  what  seemed  to  be  two  principal  deposits  of 
ore,  running  nearly  east  and  west,  and  about  a  mile  apart.  The  more 
northei-n  one,  nearest  the  lake,  has  a  total  length  of  nearlj^  a  mile,  lying  in 
sees.  28  and  27,  T.  62  N.,  R  15  W.  (p.  162). 


76  THE  VERMILION  IRON-BEARING  DISTRICT. 

WixcHELL.  N.  H.  Note  on  the  age  of  the  rocks  of  the  Mesabi  and  Vermilion 
iron  district:  Eleventh  Ann.  Rept.  Geo!,  and  Nat.  Hist.  Survey  Minn.,  for  1882,  pp. 
168-170. 

In  this  report  the  general  successiou  <  >t'  rocks  in  uortheasteru  Minnesota 
is  given  in  descending  order,  as  follows:  (1)  Potsdam,  including  the  Kewee- 
nawan  sandstones,  shales,  and  cong;lomerates,  changed  by  igneous  gabbros 
and  dolerites  locallj^  to  red  quartzites,  felsites,  quartz-porphyries,  and  red 
granites;  (2)  Taconic  g'l'oup,  including'  the  Animikie  series,  the  Gunflint 
beds,  the  Mesabi  iron  rocks,  the  Ogishke  Muncie  conglomerate  (?),  the 
Thompson  slates  and  quartzites,  and  the  Vermilion  iron  rocks;  (3)  Huronian 
group  (?),  including  magnesian  soft  schists,  becoming  syenitic  and  porphy- 
ritic,  found  .on  the  north  side  of  Gunflint  Lake,  along  the  international 
boundary,  at  Bass\yood  Lake,  and  at  Vermilion  Lake;  (4)  Montalban  (?), 
including  mica-schists  and  micaceous  granites  at  the  outlet  of  Vermilion 
Lake  and  on  the  ^Mississippi ;  (5)  Laurentian,  including  massive  horn- 
blende-gneiss and  probably  the  Watab  and  8t.  Cloud  granites. 

1SS.5. 

WiNCHELL,  N.  H.  Notes  of  a  trip  across  the  Mesabi  range  to  Vermilion  Lake: 
Thirteenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survej-  Minn.,  for  18S1, 1SS5,  pp.  20-3S. 

As  the  result  of  a  trip  from  Two  Harbors  to  Vermilion  Lake,  Winchell 
finds  between  these  two  points  two  rock  ranges,  the  first  being  the  ]\Iesabi 
proper,  and  the  second  the  Giants  range.  Resting  unconformably  upon  the 
syenites  of  the  Giants  range  are  the  Huronian  conglomerates  and  greenstones 
of  Vermilion  Lake,  while  south  of  this  range  are  the  slates  and  quartzites 
of  the  Animikie,  overlain  by  the  gabbro  and  red  granite  of  the  ^Mesabi 
range,  which  is  in  turn  overlain  by  the  trap  rocks  of  the  Cupriferous 
series.  The  Huronian  is  considered  as  resting  conformably  below  the 
Animikie,  although  not  appearing  at  the  surface.  There  are  three  iron-ore 
horizons — the  titanic  iron  of  the  gabbro  belt,  the  iron  ore  of  the  Mesabi 
range  belonging  in  the  Animikie,  and  the  hematite  of  the  Vermilion  mines, 
which  seems  to  be  the  equivalent  of  the  Marquette  and  Menominee  iron  ores. 

Several  pages  (pp.  25-35)  are  devoted  to  a  description  of  the  deposits 
exposed  by  the  stripping  operations  of  the  various  mining  companies,  and 
to  analyses  of  the  ore. 

A  few  details  are  also  given  concerning  the  distribution  and  subdivi- 
sion iif   the  cr^•stnllin('    rocks  of   Minnesota   (p]).  36-38),  from  which  we 


RESUME  OF  LITERATURE.  77 

learn,  in  addition  to  the  facts   already  presented  above,  that  below  No.  1 
the  slates  and  quartzites  of  the  Animikie  lie : 

(2)  Soft  greenish  slat_y  schists,  which  hold  lenticular  masses  of  light-colored 
protogine  gneiss,  and  also  beds  of  diorite.  The  horizon  of  the  Vermilion  iron  mines 
is  thougiit  to  be  near  the  bottom  of  this  subdivision  or  at  the  top  of  the  next,  but  en 
the  opposite  side  of  a  Laurentian  axis,  dipping  north,  and  that  of  the  Mesabi  iron 
range,  in  the  foregoing  subdivision,  dipping  south.  (.3)  Conglomeritic  and  quartz- 
itic  slates,  which  become  tine,  arenaceous  quartzites,  and  also  embrace  beds  of  sili- 
ceous marble. 

Still  further  north  [lie  the  gneiss  and  sj^enite],  accepted  ...  as  the  Laurentian. 
[pp.  37-38.] 

WiNCHELL,  N.  H.  The  crystalline  rocks  of  the  Northwest:  Proc.  Am.  Assoc. 
Adv.  Sci.,  Thirty -third  Meeting,  pp.  363-379.  Reprinted  in  Thirteenth  Ann.  Rept. 
Geol;  and  Nat.  Hist.  Survey  Minn.,  for  188i.  1885,  pp.  124-140. 

In  this  paper  Wincliell  divides  the  rocks  of  the  Northwest  into  six 
groups.  These  groups,  in  descending  order,  are  as  follows,  the  rocks  of 
the  Vermilion  range  being  assigned  to  Nos.  4,  5,  and  6:  (1)  Granite  and 
gneiss  with  gabbro;  (2)  mica-schist;  (3)  carbonaceous  and  arenaceous 
black  slates  and  black  mica-schists;  (4)  hjdromica  and  magnesian  schists, 
the  iron-bearing  horizon  at  Vermilion  Lake;  (5)  gray  quartzite  and  marble, 
which  in  Minnesota  seems  to  run  along  the  south  side  of  Ogishkie  Muncie 
Lake,  near  the  international  boundary,  including,  perhaps,  the  great  slate- 
conglomerate  which  is  there  represented ;  (6)  granite  and  syenite  with 
hornblendic  schists.  This  lowest  recognized  horizon  has  frequently  been 
styled  Laurentian.  In  Minnesota  it  is  found  on  the  international  boundary 
at  Saganaga  Lake,  and  large  bowlders  from  it  are  included  in  the  overlying 
conglomerate  at  Ogishkie  Muncie  Lake,  showing  an  important  break  in  the 
stratigraphy. 

Van  Hise,  C.  R.  Enlargements  of  hornblende  fragments:  Am.  Jour.  Sci.,  3d 
ser.,VoL  XXX,  1885,  pp.  231-235. 

Van  Hise  describes  some  enlargements  of  hornblende  fragments  and 
crystals  seen  in  the  Ogishke  Muncie  conglomerate  as  developed  on  Keke- 
kabic  Lake.  A  brief  description  of  the  macroscopic,  appearance  of  the 
conglomerate,  taken  from  W.  M.  Chauvenet's  notes,  is  also  given. 

Irving,  R.  D.  Preliminary  paper  on  an  investigation  of  the  Archean  formations 
of  the  Northwestern  States:  Fifth  Ann.  Rept.  U.  S.  Geol.  Survey,  1885,  pp.  175- 
242,  10  pis. 

In  this  paper,  published  in  1885,  Irving  gives  a  preliminary  account 
of  an  investigation  of  the  x\rchean  formations  of  the  Northwestern  States. 


78  THE  VER:\IILI0N  IRON-BEARING  DISTRICT. 

The  problems  to  be  solved  in  eacli  i-egiou  are  briefly  discussed.  Here 
reference  will  be  made  only  to  discussions  of  rocks  occurring'  in  the  disti'ict 
described  in  the  present  paper. 

It  is  maintained  that  the  series  of  rocks  first  called  Animikie  by  Hunt 
belongs  below  the  Keweenawan  rocks.  These  rocks  are  presumed  to  con- 
tinue with  interruptions  from  Thunder  Bay  along  the  boundary  to  Grunflint 
Lake,  and  thence  southwest  to  Pokegama  Falls  on  the  Mississippi. 
Throughout  a  considerable  part  of  its  extent  the  Animikie  series  of  rocks 
is  bordered  on  the  north  by  a  belt  of  granite  and  gneiss,  forming  the  Giants 
range,  to  the  north  of  which  again  come  the  strongly  folded  schists  of  the 
Vermilion  district  proper.  The  main  question  to  be  determined  here  is 
what  the  relations  of  the  Animikie  to  the  schists  inay  be.  The  hypothesis 
is  advanced  that  they  were  probably  originally  connected  over  the  inter- 
vening granite  range,  and  are  thus  really  a  unit,  which  as  the  result  of 
erosion  has  been  separated. 

Most  of  the  rocks  occurring  in  the  region  are  sedimentaries  that  have 
been  indurated  by  metasomatic  action.  With  these  are  associated  erup- 
tives,  some  of  which  have  been  so  modified  as  to  become  schists.  The 
cherty  and  jaspery  rocks  are  supposed  to  be  some  sort  of  original  chemical 
sediment,  certainly  not,  however,  the  result  of  metamorohism  of  ordinary 
sedimentary  material. 

1SS6. 

Irving,  R.  D.     Origin  of  the  ferruginous  schists  and  iron  ores  of  the  Lake 
Superior  region:  Am.  Jour.  Sci.,  3d  ser.,  Vol.  XXXII,  1SS6,  pp.  255-272. 

In  this  paper  Irving  discusses  the  origin  of  the  ferruginous  schists  and 
iron  ores  of  the  Lake  Superior  region.  An  examination  of  the  Animikie, 
Penokee,  Marquette,  Menominee,  and  Vermilion  districts  reveals  the  fact 
that  in  all  of  them  is  found  abundant  carbonate  of  iron,  which  oftentimes 
grades  into  the  othei'  foi-ms  of  the  iron-bearing  formation.  The  silica 
of  the  jasper,  actinolite,  magnetite-schists,  and  other  forms  of  the 
iron  belt  never  .show  any  evidence  of  fragmental  origin,  so  easily 
discovered  in  the  case  of  the  ordinary  quartzites  and  graywackes,  and 
is  presumed  to  be  of  chemical  origin.  Associated  witli  the  iron-bearing 
beds  is  often  a  considerable  quantity  of  carbonaceous  or  graphitic  schists. 
It  is  concluded,  (1)  that  the  original  form  of  the  iron-bearing  beds  of  the 
Lake   Superior  region  was   tliat   of  a  series  of  thinly  bedded  carbonates, 


RESUME  OF  LITERATURE.  79 

interstratified  with  carbonaceous  shaly  layers  in  places,  which  were  more 
or  less  highly  ferriferous;  (2)  that  by  a  process  of  silicification  these  car- 
bonate-bearing layers  were  transformed  into  the  various  kinds  of  ferruginous 
rocks  now  met  with;  (3)  that  the  iron  thus  removed  from  the  rock  at  the 
time  of  silicihcation  passed  into  solution  in  the  percolating  waters,  was 
redeposited  in  various  places,  and  thus  formed  the  ore  bodies  and  bands  of 
pure  oxide  of  iron;  (4)  that  in  other  places,  instead  of  leaching  out,  the 
iron  has  united  with  the  silicifying"  waters  to  form  the  silicates  now  found, 
such  as  actinolite;  (5)  that  the  silicifying  process  went  on  partly  before  the 
folding,  partly  afterward,  and  to  the  latter  period  belong  probably  the 
larg'er  bodies  of  crvstalline  ore. 

Willis,  Bailey.     Report  of  a  trip  on  the  Upper  Mississippi  and  to  Vermilion 
Lake:  Tenth  Census  Report,  Vol.  XV,  1SS6,  pp.  4-.57-ti:67. 

Willis,  in  1886,  describes  the  rocks  and  the  structure  of  a  small  part 
of  the  Vermilion  district.  The  area  surveyed  lies  along  the  south  shore  of 
Vermilion  Lake,  in  T.  62  N.,  R.  15  W.,  and  comprises  about  8  square  miles. 
One  month  was  devoted  to  the  study  of  this,  and  although  for  the  greater 
part  of  the  time  work  was  done  on  snowshoes,  many  details  were  carefully 
noted.  Traverses  were  made  one- eighth  of  a  mile  apart,  and  observations 
for  magnetic  variation  and  dip  were  taken. 

The  prominent  topographic  features  are  described  as  approximately 
east  and  west  trending  anticlinal  ridges  of  hard  jasper,  separated  by  syn- 
clinal valleys,  in  which  lie  the  younger  and  softer  rocks.  The  north  main 
ridge  or  group  of  ridges  is  known  as  the  Vermilion  range.  Southwest  of 
this,  and  separated  from  it  by  a  valley  three-fourths  of  a  mile  wide,  extends 
the  Two  Rivers  range.  Southeast  of  the  Vermilion  range  lies  Chester 
Peak  (this  is  now  known  as  Jasper  Peak),  the  west  end  of  the  thii'd  ridge. 
The  iron-bearing  series  has  a  dip  of  between  85°  and  90°,  The  succession 
from  the  base  upwards  is  as  follows:  (1)  Light-green,  thinly  laminated, 
chloritic  schist.  (2)  V^hite,  gray,  brown,  and  bright-red  jasper,  inter- 
stratified with  layers  of  hard  blue  specular  ore,  which  also  occurs  in  ore 
bodies  of  considerable  superficial  extent,  running  across  the  bedding; 
thickness  200  to  600  feet  or  more.  (3)  Chloritic  schist,  similar  to  1;  original 
thickness  probablj^  about  150  feet.  (4)  Quartzite,  dark  gray,  white,  or 
black,  of  saccharoidal  texture,  containing  grains  of  magnetite  which 
make  it  a  readily  recognized  magnetic  formation;   probable  thickness  200 


80  THE  VERMILION  IRON-BEARING  DISTRICT. 

feet.  (5)  Conglomerate,  consisting  of  sandstone  pebbles  and  traces  of  black 
slate  inclosed  in  siliceous  chloritic  schist.  (6)  Compact  homogeneous  rock, 
composed  of  quartz  grains,  chlorite,  hornblende,  plagioclase  feldspar,  and 
calcite.  This  rock  may  be  an  eruptive  quartz-diorite,  but  is  considered  a 
metamorphosed  sedimentary  transition  bed  between  5  and  7.  (7)  Black 
clay  slate,  fissile  and  sonorous.  It  occupies  a  broad  area  north  of  Ver- 
milion range.  In  section  28  huge  masses  of  jasper  form  the  crown  of 
the  arch  and  are  embedded  in  green  schist,  with  which  they  agree  in 
strike  and  dip.  The  jasper  blocks  are  rectangular  and  several  hundred 
feet  long;  the  ends  of  the  bands  come  out  squarely  to  the  contact  with  the 
schist  as  to  a  fault 

1887. 

Ieving,  R.  D.  Is  there  a  Hm-onian  group?:  Am.  Jour.  Sci.,  3d.  ser..  Vol. 
XXXIV,  1887,  pp.  20Jr-216,  249-263.  365-37i. 

In  these  papers  Irving  discusses  the  separability  of  a  Huronian  group 
from  an  underlying  series  and  demonstt-ates  the  possibility  of  such  separation 
in  the  original  Huronian  region,  in  the  Marquette  and  Menominee  districts 
of  Michigan,  in  the  Penokee  district  of  Wisconsin  and  i\Iicliigan,  and  finally 
in  the  Vermilion  Lake  region  of  Minnesota  and  Canada  (Ontario).  In  the 
Vermilion  region  the  gently  tilted  Animikie  series  of  slates,  gray  wackes,  and 
iron-bearing  rocks,  with  interstratified  sheets  of  diabase  and  gabbro,  resem- 
bles very  strongly  in  hthologic  aspect  the  Penokee  series  of  the  south 
shore  and  rests  in  palpable  unconformity  upon  a  folded  series  of  schists, 
granites,  and  gneisses.  Above  it  is  the  Keweenaw  series,  which  bears  the 
same  unconformable  relations  to  the  underlying  rocks  as  it  does  to  the 
Penokee  series. 

Thus  the  Animike  series  occupies  very  pLainl.y  the  stratigraphical  position  of  the 
original  Huronian  and  of  the  various  iron-bearing  groups  of  the  south  shore  of  Lake 
Superior.  Since  it  is  also  intrinsicallj^  so  extraordinarily  like  the  Penokee  series  as 
to  leave  no  doubt  of  their  identity,  and  since  the  Penokee  is  as  evidently  the  equiva- 
lent of  the  original  Huronian,  we  seem  to  be  left  no  choice  as  to  calling  the  Animike 
Huronian  also  [p.  263]. 

North  of  tlie  Animikie  beds  are  schistose  iron-bearing  rocks,  which 
extend  from  Vermilion  Lake  to  the  vicinity  of  Knife  and  Saganaga  lakes. 
These  are  flanked  by  gneisses  and  granites,  and  on  account  of  their  litho- 


RESUME  OF  LITERATURE.  81 

logic  similarity  to  the  Animikie  rocks  are  taken  to  be  their  folded  equivalent. 
While  there  is  not  here  the  same  palpable  unconformities  as  in  the  other 
regions  discussed,  it  is  believed  that  there  are  two  gTOups  of  rocks,  one  of 
crystalline  schists,  and  another  of  newer  detrital  rocks,  the  apparent 
unconformity  between  these  being  due  to  the  intense  folding. 

WiNCHELL,  A.  Report  of  geological  observations  made  in  northeastern  Minne- 
sota during  the  season  of  1SS6:  Fifteenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey 
Minn.,  for  1886,  1887,  pp.  5-307. 

In  this  report  are  given  the  detailed  observations  made  on  an  extensive 
trip  in  northeastern  Minnesota. 

The  region  presents  a  series  of  schists  flanked  on  the  north  and  south 
by  massive  crystalline  rocks.  In  the  western  part  of  the  district  these 
rocks  are  gneissic  on  both  sides,  but  in  the  eastern  part  the  schists  extend 
on  the  north  beyond  the  limits  of  the  map,  while  on  the  south  the  gneissic 
rocks  are  replaced  by  gabbro  and  greenstone.  The  schists  and  bedded 
crystallines  stand  in  nearly  vertical  attitude. 

It  is  said  that  the  indications  of  a  genetic  connection  between  graywacke 
and  the  mica-schists  are  very  noteworthy,  a  gradation  from  one  to  the  other 
having  been  noted  in  numerous  instances  (p.  176).  Likewise  the  schists 
grade  into  the  gneissic  rocks,  there  being  nowhere  an  abrupt  passage  from 
one  class  to  the  other.  In  the  passage  from  the  schists  to  the  gneisses  there 
is,  first,  an  increase  in  frequency  of  ramifying  veins,  next,  lumps  of  gneiss 
or  granite  occur  in  the  schists,  and  finally  there  is  interstratification  of  the 
schists  and  gneisses  (p.  178).  The  author  expresses  himself  (p.  179)  as 
uncertain  whether  or  not  the  conglomerate  at  Ogishkie  Muncie  Lake,  which 
attains  an  enormous  development  and  contains  varieties  of  granitic  and 
quartzose  bowlders,  as  well  as  flint,  jasper,  porphyry,  and  greenstone,  exists 
as  far  west  as  Vermilion  Lake;  however,  there  is  apparently  no  doubt  that 
it  lies  in  the  strike  of  the  schists  occurring  at  the  west  end  of  the  range  on 
Vermilion  Lake.  It  thus  seems  to  be  a  local  development  of  the  schists. 
The  beds  of  bowlders  are  interbedded  with  flinty  argillites,  wliich  attain 
their  greatest  development  north  of  the  conglomerates;  the  southern  border 
of  the  conglomerates  is  concealed  by  overlying  greenstone  and  gabbro. 
Some  sericitic  beds  have  been  discovered  within  the  conglomerate  formation. 
These  facts  lead  the  author  to  conclude  that  the  conglomerate  belongs  in 
stratigraphic  position  within  the  northern  border  of  the  sericitic  schists, 

MON  XLV — 03 6 


82  THE  VERMILION  IRON-BEARING  DISTRICT.       , 

and  the  southern  border  of  the  argilhtes  as  they  appear  farther  west.  The 
entire  SA^stem  of  g-neisses,  schists,  and  slates  is  regarded  as  belonging  to  one 
structural  system,  as  they  all  possess  a  common  dip  and  pass  by  gradations 
into  each  other,  both  along  the  strike  and  across  it  (p.  181).  The  iron- 
bearing  rocks  are  interlaminated  with  the  country  schists,  and  while  they 
exhibit  much  persistence  in  the  direction  of  the  strike,  they  do  not  continue 
without  interruption;  they  appear  in  the  midst  of  the  schists  sometimes  as  a 
strictly  local  phenomenon  (p.  182).  In  structure  the  region  is  a  simple 
synclinical  fold,  the  strata  of  which  have  a  thickness  of  106,204  feet.  The 
succession,  from  the  bottom  upward,  is  granite,  gneiss,  micaceous  and 
hornblendic  schists,  graywacke,  argillite-schist  bearing  conglomerates,  and 
sericitic  and  chloritic  schists  bearing  iron  ore  (p.  191).  As  the  plainly 
fragmental  rocks  grade  by  imperceptible  stages  into  the  gneiss  and  the 
gneiss  into  the  granite,  the  whole  is  regarded  as  a  sedimentar}''  series  (p.  193). 
While  granite  pebbles,  are  found  in  the  conglomerates,  these  are  not  derived 
from  the  underlying  granite,  as  many  of  the  fragments  diflPer  in  character 
from  the  inferior  granite  (p.  194). 

The  author  places  the  conglomerates  stratigraphically  below  the  iron 
ores  and  jaspers.     He  makes  the  following  statement: 

We  find  flints  and  jaspers,  which,  as  far  as  we  have  explored,  could  not  be 
afforded  by  an}^  part  of  this  system.  We  find  nothing  which  indisputably  could 
have  been  derived  from  an}'  member  of  the  system — the  Vermilion  S3'stem — ranging 
from  the  granites  to  the  earth}'  schists.  Those  older  rocks  whose  destruction  afi'orded 
material  for  the  building  of  the  Vermilion  system  belonged  to  an  earlier  age,  and 
were  parts  of  an  older  system  [pp.  19-1-19.5]. 

They  all  belong  to  one  system,  for  no  grounds  were  discerned  which 
would  justify  the  division  of  this  series  of  rocks  into  several  systems 
(p.  195). 

WiNCHELL,  N.  H.  Geological  report:  Fifteenth  Ann.  Rept.  Geol.  and  Nat. 
Hist.  Survey  Minn.,  for  1886,  1887,  pp.  209-399,  with  map. 

In  this  report  Winchell  gives  very  numerous  details  as  to  the  geology 
of  northeastern  Minnesota.  A  preliminary  geologic  map  accompanies  the 
report.  At  several  places  there  are  transitions  between  the  granite-gneiss 
and  a  fine-grained  mica-schist.  In  the  syenite  are  sometimes  found 
angular  fragments  of  mica-schist.  The  Vermilion  group  is  defined  as 
iiichiiling  the  lower  portion  of  the  complex  series  of  schists  designated  as 


RESUME  OE  LITERATURE.  83 

Keewatin  by  Lawsou.  It  embraces  the  mica-schists  and  hornblende- schists 
of  Vermilion  Lake  and  their  equivalents,  and  lies  between  the  graywackes 
on  the  one  side  and  the  basal  syenites  and  granites  on  the  other. 

Adjacent  to  Vermihon  Lake  are  hematite  ores  associated  with  jasper, 
which  are  inclosed  in  a  greenish  magnesian  schist,  the  bedding  of  which 
stands  vertical.  This  schistose  rock  is  probably  of  igneous  origin,  and  in 
its  relations  to  the  jasperoid  rocks  it  fills  all  their  cavities,  overlying  them 
unconformably,  and  holding  fragments  of  the  jasper,  all  indicating  its  later 
origin.  This  igneous  rock  passes  into  a  chlorite- schist,  and  this  into  the 
sericite-scliists  and  graywackes,  which  show  unmistakable  evidence  of  an 
aqueous  arrangement  (pp.  219-221).  The  jasperoid  is  a  sedimentary  rock 
(p.  245  et  seq.)  and  not  an  eruptive,  as  has  been  supposed  by  Wadsworth. 
The  rock  was  not,  however,  deposited  in  its  present  condition.  The  beds 
have  been  upturned,  folded,  crushed,  and  affected  by  intense  chemical 
action.  The  ore  is  regarded  as  a  result  of  chemical  or  metasomatic  change. 
The  ore  is  a  hard  hematite  and  of  such  good  quality  as  to  warrant  a  guar- 
antee of  67  per  cent  or  more  of  iron,  and  0.06  j^er  cent  or  less  of  phosphorus. 
The  general  succession  from  above  downward  is  as  follows:  (1)  Gabbro. 

(2)  Diabasic  dolerite.     These  rest  unconformably  upon  the  lower  members. 

(3)  Reddish  gneiss  and  syenite,  which  includes  the  Misquah  Hills,  White 
Iron  Lake  and  the  Giants  range  (Mesabi  heights).  This  is  a  ease  of  a 
fusion  of  sedimentary  beds  in  situ,  although  it  is  not  generally  complete. 

(4)  Graywacke,  sericite-schist,  argillite,  quartzite,  and  jaspilite,  which  occur 
about  Vermilion  Lake.  (5)  Mica-schist,  hornblende-schist,  and  diorite — the 
Vermilion  group.  (6)  Mica-schist  and  granite,  veined  with  syenite  and 
granulite.  (7)  Lower  syenites  and  gneisses,  generally  regarded  as  Lauren- 
tian.  Nos.  3  to  7  are  conformable,  and  Nos.  4  to  7  graduate  into  each  other 
(p.  355). 

The  author  states  it  as  his  opinion  (p.  356)  that  there  is  reason  for 
believing  that  the  Animikie  rocks  overlie  the  greenstone  (No.  2)  and 
underlie  the  gabbro  (No.  1)  of  the  above  succession. 

18SS. 

WiNCHELL,  N.  H.    Sixteenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minn., 
for  1887,  1888.  pp.  13-129. 

Winchell,  as  customary,  publishes  in  the  annual  report  his  field  notes 
for  the  preceding  ye-a.v,  giving  an  abundance  of  details  about  the  rocks  of 


84  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  Vermilion  district.  As  a  result  of  a  traverse  made  into  Canadian  ter- 
ritory from  the  east  end  of  Gunflint  Lake,  he  comes  to  the  conclusion  that 
the  Animikie,  on  Gunflint  Lake,  while  not  found  in  exact  superposition  on 
the  Keewatin,  bears  such  relation  as  to  render  it  probable  that  the  t^Y0 
formations  are  discordant.  A  short  distance  north  of  the  xinimikie  the 
Keewatin  rocks  are  found  with  a  dip  of  80°  south,  and  these,  a  little  farther 
to  the  north,  grade  conformably  into  micaceous  and  hornblendic  schists. 
He  continues  as  follows: 

In  the  report  of  1881,  p.  95,  .  .  .  some  reasons  are  given  for  considering  the 
"quartzite-slate  formation"  seen  at  Gunflint  Lake  (the  Animike)  in  horizontal  posi- 
tion, the  equivalent  of  the  great  quartzite  and  slate  forniatiou  at  Ogishke  ISIuncie 
Lake,  which  passes  into  the  Ogishke  conglomerate.  .  .  .  At  that  time  the  tilted  schists 
and  graA'wackes  of  the  Kewatin  series,  with  their  contained  iron  ore,  were  con- 
sidered an  integral  part  of  the  same  tilted  series  as  the  slates  and  quartzites  associated 
conformably  with  the  Ogishke  conglomerate,  and  the  iron  ore  of  the  jasper  ridges 
at  Vermilion  Lake  were  considered  the  equivalent  of  the  iron  ore  seen  in  the 
Animike.  .  .  .  But  since  the  separation  of  the  Animike  from  the  Kewatin  has  been 
established  by  mai'ked  unconformities,  and  by  constant  differences  in  lithology 
(including  a  constant  difference  in  the  kind  of  iron  ore  associated  and  their  respective 
mineral  accompaniments),  it  remained  still  to  answer  the  question.  To  which  series, 
the  Animike  or  the  Kewatin,  does  the  quartzite-slate  conglomerate  of  Ogishke 
Muncie  Lake  belong^     [p.  79.] 

An  attempt  was  made  to  trace  the  Animikie  westward  and  get  its 
relations  to  the  rocks  on  Ogishke  Muncie  Lake.  The  Animikie  rocks 
rest  unconformably  on  the  gneiss  west  of  Gunflint  Lake.  The  Pewabic 
quartzite  is  a  magnetited  rock  apparently  near  the  top  of  the  Animikie. 
The  gabbro  is  observed  overlying  the  Animikie  (Pewabic  quartzite)  at 
many  places.  The  Animikie  lies  unconformably  on  the  Keewatin  north 
of  Gunflint  Lake  (p.  87).  In  passing  from  Gunflint  Lake  the  Animikie 
is  found  to  have  a  dip  varying  from  12°  to  65°  SSE.  At  Gabemichigama 
Lake  a  gradation  is  supposed  to  exist  from  the  flat-lying  Animikie  into  the 
Ogishke  Muncie  conglomerate,  with  interstratified  quartzite  and  slate  beds 
sti'iking  northwest  and  dipping  88°  northeast  (p.  91). 

Studies  made  around  Ogishke  Muncie  Lake  show  that  the  Ogishke 
conglomerate  can  be  divided  into,  first,  an  old,  eruptive-looking,  massive 
schistose  and  decayed  conglomerate,  which  belongs  to  the  Keewatin,  and 
extends  from   Stuntz   Island,  in  Vermilion   Lake,  past   Fdy  (wliere  it  was 


RESUME  OF  LITERATURE.  85 

recently  examined  at  the  iron  mines)  to  Twin  Mountain  and  Frog  Rock 
Lake;  second,  the  Ogishkie  conglomerate  proper  of  later  date,  fresher 
aspect,  and  more  siliceous,  evidently  derived  largely  from  the  disintegration 
of  the  other,  upon  which  it  lies  unconformably.  With  this  second  phase 
the  Animikie  quartzite  and  slates  are  interstratified  (p.  98). 

Partly  surrounding  Cacaqiiabic  Lake  is  found  a  green  schist  Avhich 
belongs  apparently  to  a  date  about  the  same  as  the  Keewatin  portion  of 
the  Ogishke  conglomerate  or  is  its  immediate  successor  and  conformable 
upon  it.  Nevertheless  they  are  markedly  different.  The  green  schist  is 
apparently  formed  of  basic  erupted  materials  in  a  fragmental  condition  and 
received  its  stratified  arrangement  through  the  agency  of  water.  Volcanic 
vents  in  the  immediate  neighborhood  must  have  given  origin  to  this  vast 
supply  of  basic  materials  (p.  108). 

It  is  concluded  that  a  great  basic  eruption  separated  the  Animikie  and 
Keewatin,  as  shown  by  this  volcanic  fragmental  material,  as  well  as  by  the 
existence  of  mountains  of  g-reenstone,  which  are  to  be  regarded  as  the 
probable  sources  of  the  fragmental  rock  (p.  108). 

WiNCHELL,  A.  Sixteenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minn.,  for 
1887, 1SS8,  pp.  133-391.  Unconformability  between  the  Animikie  and  the  Vermilion 
series:  Am.  Jour.  Sci.,  3d  ser.,  Vol.  XXXIV,  1887,  p.  Sltt.  See  also:  The  uncon- 
formities of  the  Animikie  in  Minnesota:  Am.  Geologist,  Vol.  1, 1888,  pp.  14-24.  Two 
sj'stems  confounded  in  the  Huronian:  Am.  Geologist,  Vol.  Ill,  1889,  pp.  212-214:, 
389-340.  Systematic  results  of  a  field  study  of  the  Archean  rocks  of  the  Northwest: 
Proc.  Am.  Assoc.  Adv.  Sci.,  Thirty-seventh  Meeting,  1888,  pp.  205-206.  The  geo- 
logical position  of  the  Ogishkie  conglomerate:  Proc.  Am.  Assoc.  Adv.  Sci.,  Thirty- 
eighth  Meeting,  1889,  pp.  234-235. 

In  the  above  papers  Alexander  Winchell  reports  that  he  finds  upon 
Wonder  Island,  in  Lake  Saganaga,  a  conglomerate  which  contains  abundant 
rounded  pebbles  in  a  groundmass  of  syenite  (p.  219).  This  occurrence 
gives  rise  to  the  following  suggestion  in  the  author's  mind: 

The  inferences  from  the  occurrence  are  important.  A  pudding  stone  like  this 
is  universally  regarded  as  of  fragmental  origin.  Not  onlj^  that,  but  of  origin 
through  aqueous  agency.  .  .  .  So,  if  this  conglomerate  is  sedimentary  in  nature, 
the  S3^enite  groundmass  must,  at  the  time  of  the  deposition  of  the  pebbles,  have 
been  also  in  a  state  of  semifluiditj'  under  the  influence  of  water.  It  may  have  been 
subjected  simultaneously  to  energetic  thermal  action;  but  it  was  not  in  that  state  of 
fluidity  which  accompanies  and  results  from  recent  eruption  as  molten  matter  from 


86  THE  VERMILION  IRON-BEARING  DISTRICT. 

some  deep  source.  This  view  of  the  orig-jii  of  granitic  rocks  I  have  heretofore 
maintained,  and  this  remarkable  observation  is  a  gratifj'ing  confirmation  of  the 
correctness  of  the  opinion  [p.  219]. 

The  lower  limit  of  the  conglomerate  is  very  abrupt,  and  the  conglom- 
erate is  figured  as  overlying  the  syenite.  Nevertheless  it  is  concluded 
that — 

The  epoch  of  the  paste  [in  which  the  pebbles  lie]  and  that  of  the  deposition  were 
the  same.  The  conglomerate  and  the  syenite  were  put  in  place  simultaneously.  The 
syenite  was  not  "erupted"  after  the  conglomerate  existed.  The  conglomerate  was 
not  laid  down  on  the  solidified  syenite  [p.  321]. 

On. the  north  side  of  Gimflint  Lake  he  finds  argillites  standing  nearly 
vertical. 

The}'  are  not  at  all  ambiguous.  They  are  the  Knife  Lake  slates  preserving  to 
this  point  their  steady  yerticalitj',  and  here  remaining  uncovered  by  Animike. 
This  looks  like  a  solution  of  a  vexed  problem.     The  Animike  and  the  Vermilion 

slates  are  not  one  .  .  .  The  dip  [of  these  slates]  is  S.  89*^'.  The  strike  of  the  sheet  is 
N.  72^  E.  [p.  253]. 

The  Animikie  .slates  are  found  resting  unconformably  upon  A-ertical 
schists,  gneisses,  and  syenites  at  several  points  on  Gunflint  Lake  [p.  323]. 
On  the  west  side  of  West  Sea  Gull  Lake  the  conglomerate  and  syenite  are 
reported  as  interbedded.  This  conglomerate  is  thought  to  be  comparable 
with  that  of  Wonder  Island  (p.  293).  On  the  north  side  of  Sea  Gull  Lake 
the  syenite  contains  sharply  limited  rounded  pebbles  and  irregular  masses 
of  hornbleudic  and  diabasic  material  (p.  298).  On  Epsilon  Lake  the  argil- 
lite  has  schistic  planes  standing  vertical,  while  the  bedded  structure  has  a 
dip  of  only  23°  toward  S.  40°  W.  (p.  322). 

From  the  summary  of  facts  concerning  the  region  we  learn: 

Within  the  region  here  considered  the  geographical  distribution  of  the  several 
terranes  is  east-northeast  in  the  Vermilion  district  and  nearly  east  in '  the  district 
eastward  from  Knife  Lake.  Throughout  the  entire  region  the  clastic  rocks — not 
excluding  the  so-called  granites  and  sj'enites,  present  a  liedded  structure — sometimes 
indeed  obscure,  but  everywhere  discernible  over  all  considerable  exposures.  Among 
the  granites  and  gneisses  the  bedding  may  possibly  lie  regarded  as  the  result  of 
foliation  alone;  Init  I  have  been  led  to  think  that  its  direction  was  predetermined  l)y 
pianos  of  sedimentation.  .  .  .  For  similar  reasons,  I  regard  the  bedding  of  the 
crystalline  schists  as  primitively  sedimentary.  The  two  older  systems  of  rocks  ha\-e 
their  planes  of  bedding  nearly  vertical — inclining  only  a  few  degrees  in  one  direction 


RESUME  OF  LITERATURE.  87 

or  the  other.     The  bedding  of  the  newer  system  is  nearly  horizontal — inclining  five 
to  fifteen  degrees  southward  in  the  regions  here  reported  on  [p.  330]. 

811111101110'  up,  we  get  the  following-  succession  (pp.  330-364): 

1.  At  the  base  are  the  r/ranifoid  and  gneissoid  rocks  in  three  areas — the 
Basswood,  White  Iron,  and  Saganaga  lakes.  The  White  Iron  granite  area 
is  made  to  include  the  area  on  and  near  Snowbank  Lake  that  is  underlain 
by  granite.  These  granitic  masses  have  everywhere  a  bedded  structure, 
more  or  less  distinct.  They  are  traversed  by  quartzose  and  g-ranulitic 
veins,  as  well  as  by  dikes  of  diabase. 

2.  The  gneisses  and  granites  are  flanked  by  vertical  crystalline  schists  of 
the  Vermilion  group.  The  transition  from'  the  gneisses  to  the  crystalline 
schists  is  never  abrupt,  but  is  a  structural  gradation,  near  the  line  of 
junction  the  beds  of  gneisses  and  schists  occurring  in  many  alternations. 

3.  Above  the  Vermilion  group  are  the  Keivatin  semicrystaUine  schists, 
the  two  series  being  everywhere  conformable;  but  there  is  a  somewhat 
abrupt  change  from  one  group  to  the  other,  and  this  indicates  the  possibility^ 
of  an  original  unconformity.  If  such  an  unconformity  existed,  as  is 
thought  improbable,  it  has  been  destroyed  by  laterial  pressure.  There  has 
been  no  actual  connection  traced  between  the  Kewatin  schists  north  of 
Grunflint  Lake  and  those  of  Knife  Lake.  The  Kewatin  schists  are  almost 
everywhere  vertically  bedded.  When  the  bedding  is  obscure  this  is  some- 
times due  to  the  action  of  erupted  masses,  but  more  often  the  metamor- 
phosed condition  of  the  strata  is  not  ascribable  to  any  visible  cause.  The 
Kewatin  schists  include  graywacke,  argillite,  sericite-schist,  chlorite-scliist, 
porphyrellite-schist,  and  hematite.  The  Ogishke  conglomerate  is  placed 
as  part  of  the  Kewatin  system,  as  it  is  traced  liy  actual  gradations  into 
the  adjoining  argillites.  These  argillites  and  associated  schists  are  in 
continuity  with  the  argillites  and  schists  of  Vermilion  Lake,  ^vliile  in  the 
conglomerate  itself  are  local  developments  of  sericite-schist.  The  .bedding 
of  the  conglomerate  is  nearly  vertical;  its  pebbles  are  metamorphosed; 
they  include  numerous  varieties,  among  which  are  syenite  resembling  the 
Saganaga  syenite,  greenstone,  porphyry,  red  jasper,  flint,  quartz,  petrosilex, 
ordinary  syenite,  diorite  (coarse  and  fine),  porphyroid,  siliceous  schist,  and 
carbonaceous  siliceous  argillite.  On  structural  as  well  as  lithologic 
grounds  the  Ogishke  conglomerate  seems  to  be  part  of  the  Kewatin, 
although  there  are  some  reasons  for  suspecting  it  to  belong  to  the  Animikie. 


88  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  Ogishke  conglomerate  is  not  to  be  confounded  with  the  Stuntz 
conglomerate,  which  occupies  a  lower  stratigraphic  position  than  does  the 
Ogishke.     That  the  Kewatin  schists  are  eruptive  is  regarded  as  improbable. 

4.  The  Animikie  series,  resting  uncouformably  upon  the  Keewatiu, 
stretches  from  Thunder  Bay  as  far  as  Duluth  and  still  beyond  to  the 
Mississippi,  and  perhaps  includes  some  of  the  slates  as  far  west  and  north 
as  Knife  Lake.  The  Animikie  formation  is  generally  in  a  nearly  horizontal 
position,  the  dip  not  being  more  than  from  5°  to  15°  SSE.  The  formation 
is  essentially  an  argillite,  which  embraces  jaspery  magnetitic,  hematitic, 
and  sideritic  beds.  At  Gabimichigama  Lake  the  Animikie,  represented 
by  the  "muscovado,"  is  in  its  characteristic  horizontal  position,  while  the 
vertically  bedded  terrane  underlies  it.  The  sedimentary  series  is  cut  by 
dikes  of  igneous  rock,  determined  macroscopically  as  diabase-norite  and 
porphyry. 

For  the  system  of  semicrystalline  schists  subjacent  to  the  Animikie,  to 
which  the  term  Kewatin  has  been  applied,  Marquettian  is  pi'oposed.  The 
succession  of  terranes  in  northeastern  Minnesota  is,  in  descending  order, 
summarized  as  follows  (pp.  366-367): 

HuKONiAN  SYSTEM  (compai'c  scc.  2  of  this  report"),  over  4,082  feet. 
Magnetitic  group.     32  feet. 

Dark,  laminated,  shaly  argillite,  sometimes  magnetitic,  29  feet. 

Magnetitic  lieds,  often  uppermost,  8  feet.     Place  of  sideritic  bed? 

Muscovado,  uppermost  when  the  two  above  are  wanting,  4  feet. 
Siliceous  group.     50  feet. 

Siliceous  argillites  and  siliceous  and  jaspery  schists,  50  feet. 
Argillitic  group.     4,000  feet. 

Dark,  laminated,  shaly  argillites,  over  4,000  feet  in  Minnesota. 

(Bottom   of    the   .sj'.stem    not    reached  at    contacts   seen   with   gneiss  and 
Marquettian.) 
Marquettian  system.     27,500  feet. 

Ogishke  group.     10,000  feet,  but  local.     (Perhaps  half  this). 

Ogishke  conglomerate,  slaty  and  diabasic.     4,500  feet  each  side  of  synclinal. 

Ogishke  dolomite,  included  in  the  conglomerate,  10  feet. 

Conglomerate  greenrock.     500  feet  each  side  of  synclinal. 
Tower  group.     (Earthy  schists.)     15,000  feet. 

Sericitic  and  argillitic  schists,  with  beds  of  hematite,  5,OU0  feet. 

(These  sometimes  changed  to  chloritic  .schists.) 

"This  refers  to  Sixteenth  Ann.  Kept.  Minn.  Geol.  and  Nat.  Hist.  Survey. 


RESUME  OF  LITERATURE.  89 

Marquettian  system — Continued. 
Tower  group — Continued. 

(The_v  pass  eastward  into  schists  prevailingly  porphyi'ellitic.) 
Stuntz  conglomerate,  porodyte  and  porphyrel,  20  feet. 
Graywaclve  group.     2,500  feet. 
Gra3'wacke  and  horufels. 

Graywacke  with  indications  of  fine  mica  and  hornblende.     ("Nascent  mica 
schists.") 

Laurentian  system.     89,500  feet. 

Vermilion  group.     Over  1,500  feet. 

Crystalline  schists — micaceous,  hornblendic,  dioritic,  granulitic. 
Gneissic  group.     Over  88,000  feet. 

Chlorite-gneiss.     (Not  universally  developed.) 

Saganaga,  White  Iron,  and  Basswood  gneisses. 

Thus  the  crj-stalline  schists  and  gneisses  fall  entireh'  within  the  Laurentian 
system.  There  are  no  Huronian  gneisses  in  Minnesota.  We  find  nothing  of  ' '  older  " 
and  "newer"  gneisses.  We  find  no  ''claj'  slates"  beneath  the  horizon  of  the 
crystalline  schists.  But  I  can  not  deny  the  existence  of  a  different  state  of  things  in 
other  regions.  To  me  it  seems  probable,  however,  that  a  comparative^  undisturbed 
region  like  northeastern  Minnesota  must  approach  near  to  a  normal  exhibit  of  the 
real  sviccession  of  the  Archean  rocks. 

Irving,  R.  D.  On  the  classification  of  the  early  Cambrian  and  pre-Cambrian  for- 
mations: Seventh  Ann.  Rept.  U.  S.  Geol.  Survey,  1888,  pp.  365-45i,  with  22  plates 
and  maps. 

Irving,  in  1888,  discusses  the  classification  of  the  early  Cambrian  and 
pre-Cambrian  formations,  and  particularly  those  of  the  Northwestern  States. 
The  relations  of  the  Animikie,  Penokee,  Marquette,  Menominee,  and  Vermil- 
ion Lake  iron-bearing  series  to  the  underlying  and  overlying  series  are  again 
fully  discussed.  The  Keweenawan  is  held  to  overlie  the  Huronian  every- 
where by  a  very  considerable  unconformity.  At  the  base  of  the  Keweenawan 
is  a  great  mass  of  gabbro,  which  extends  from  Duluth  northeast  to  the  inter- 
national boundary,  more  than  100  miles,  and  at  its  maximum  is  more  than  20 
miles  wide.  This  basal  gabbro  is  now  in  contact  with  one  member  of  the 
Animikie,  and  now  with  another,  while  in  other  places  it  is  in  contact  with  the 
lower  crystalline  schists  or  granite.  In  the  Huronian  are  placed  the  original 
Hiu-onian,  the  Iron-bearing  series  of  Michigan  and  Wisconsin,  the  Black 
River  Falls  iron-bearing  series,  the  Animikie  series,  the  St.  Louis  and 
Mississippi  slate  series,  the  Vermilion  Lake  iron-bearing  series,  the  Baraboo 


90  THE  VERMILION  IRON-BEARING  DISTRICT. 

quartzite  series,  and  the  Sioux  quartzite  series.  Under  the  Hurouian  is  the 
Laurentian,  separated  from  it  by  a  great  iincouformity.  This  is  a  series  of 
D-i-anites,  gneisses,  hornblende-schists,  mica-schists,  and  other  green  scliists. 

1889. 

Hall.  C.  W.  The  di.stribution  of  the  g-ranites  of  the  Northwestern  States,  and 
their  general  lithologic  characters:  Proc.  Am.  Assoc.  Adv.  Sci.,  Thirty-seventh 
Meeting,  1889,  pp.  225-226. 

In  the  above  paper  Hall  describes  the  distribution  of  the  granites  of 
the  Northwestern  States,  particularly  those  of  Minnesota. 

In  Minnesota  they  occur  (1)  in  several  belts  along  the  Canadian 
boundary  projecting  southwesterly  into  the  State;  (2)  as  quite  prominent 
masses,  wliether  connected  with  those  along  the  boundary  or  not,  around 
Vermilion,  Snowbank,  and  other  lakes;  (3)  forming  the  Mesabi  or  Giant 
range;  (4)  at  a  number  of  places  in  the  central  part  of  the  State.  These 
are  found  to  be  either  intrusive  or  granitic  vein-stones,  the  latter  iDeing 
insignificant  in  quantity.  Tlie  granites  of  Minnesota  as  to  age  are  probably 
later  than  the  Laurentian  floor  of  the  continent,  but  earlier  than  those  of  the 
Agnotozoic  era. 

They  belong  to  one  of  tlie  three  or  four  grand  periods  of  ei-uptive 
activity  determinable  in  the  Northwestern  States. 

WiNCHELL,  N.  H.  Seventeenth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  ^Nlinn., 
for  1888,  1889,  pp.  5-74;  see  also  the  Animikie  black  slates  and  quartzites,  and  the 
Ogishki  conglomerate  of  Minnesota,  the  equivalent  of  the  '^ Original  Huronian:" 
Am.  Geologist,  Vol.  I,  1888,  pp.  11-14;  also  Methods  of  stratigraphy  in  studying  the 
Huronian:  Am.  Geologist,  Vol.  IV.  1889.  pp.  342-357. 

In  this,  Wincliell  gives  a  review  of  the  work  done  upon  the  crystal- 
line rocks  of  northeastern  Minnesota  by  the  State  survey,  showing  the 
progression  in  the  ideas  held  concerning  their  characters  and  stratigraphy 
(pp.  6-28).  Then  follows  a  summary  of  the  results  of  the  investigations 
as  they  appear  up  to  that  time  (pp.  28-74).  In  many  points  the  conclusions 
and  facts  are  the  same,  of  course,  as  in  the  previous  reports.  The  Lauren- 
tian age  (pp.  28-31)  is  made  to  include  the  gneiss,  granitic,  and  syenitic,  Init 
excludes  the  crystalline  schists.  It  is  the  fundamental  gneiss  of  iMinnesota. 
"It  resulted  from  the  fusion  and  recrystallization  of  the  earliest  sedi- 
ments" (p.  28). 


RESUME  OF  LITERATURE.  91 

The  Laurentian  g-neiss  is  represented  in  the  Vermilion  district  by  the 
Basswood  Lake  and  pei'haps  the  Saganaga  Lake  g-ranite.     However: 

There  may  be  spots,  or  considerable  areas,  within  this  original  gneissic  belt, 
where,  bj'  subsequent  deeij-seated  hj^drothermal  fusion,  these  primitive  Laurentian 
sediments  have  been  rendered  plastic  and  then  fluid,  and  have  b}'  pressure  been 
extended  through  fissures  in  the  crust  to  the  surface  or  have  been  uncovered  as  lacco- 
lites  b}'  the  destruction  of  the  overlying  strata;  but  wherever  these  exist  the}'  are 
presumed  to  show  their  later  origin  by  their  nongneissic  structure,  or  by  their 
overlying  some  later  sedimentary  strata.  The  distinction,  howe\-er,  between  the 
eruptive  condition  of  the  fused  Laurentian  sediments  and  the  primitive  sediments 
that  have  been  converted  in  situ  into  the  fundamental  gneiss  is  one  that  requires 
more  study  before  it  can  be  defined.  That  both  conditions  exist  there  can  be  no 
question;  that  they  can  always  be  distinguished  is  not  to  be  afiirmed  [p.  29]. 

Closely  associated  with  the  belts  of  fundamental  gneiss  are  areas  of 
massive  erruptive  syenite  which  have  resulted  from  such  hydrothei-mal 
fusion  of  the  gneiss.  Such  syenite  is  found  north  of  Gunflint  Lake,  on  the 
shores  of  Cacaquabic  Lake. 

The  Laurentian  gneisses  are  seen  at  places  to  be  conformable  with, 
and  to  grade  into  the  hornblendic  and  micaceous  "crystalline  schists" — the 
Vermilion  schists,  which  are  the  equivalents  of  Lawson's  Coutchiching.  At 
other  places  the  gneisses  and  schists  are  uncomfoi'mable  and  here  they  both 
play  the  r61e  of  eruptive  rocks  interpenetrating,  in  the  form  of  transverse 
dikes,  and  inclosing  fragments  of  each  other.  This  relationship  is  con- 
sidered to  be  eA'idence  of  volcanic  action.  "It  is  manifest,  therefore,  that 
the  supposition  of  the  advent  of  a  characteristically  eruptive  era,  closing 
the  quiet  Laurentian  sedimentary  age,  will  account  for  both  an  unconform- 
able and  a  conformable  transition,  such  as  are  seen,  from  the  Laurentian  to 
the  Vermilion"  (p.  35). 

The  Vermilion  group  passes  by  conformable  transition  into  the  Keewa- 
tin.  The  character  of  the  Keewatin  rocks  indicates  that  there  was  active 
volcanic  action  during  the  whole  period,  and  that  the  ejectamenta  were 
received  and  distributed  by  the  w^aters  of  the  suiTOunding  sea.  This  is 
indicated  by  the  alternation  of  the  breccias  and  volcanic  material  with 
truly  sedimentar)-  strata.  The  name  Kawishiwin  is  proposed  for  the 
massive  greenstone  stage  of  the  Keewatin.  The  Keewatin  is  the  iron- 
bearing  formation.  The  iron  ore  is  associated  with  the  jaspilite,  which 
is  of  a  sedimentary  orig-in.     Above  the   Keewatin  is   a  profound  uncon- 


92  THE  VERMILION  IRON-BEARING  DISTRICT. 

foriiiity  (pp.  37-46).     Above  this  lies  the  Aniinikie.     This  has  the  Ogishke 
conglomerate  as  its  base. 

This  coiii^loinerate  is  followed  by  an  immense  thickness  of  dark  slat^'  rocks, 
often  chert}',  or  flinty,  frequently  very  dark-colored,  generally  siliceous,  alternating 
with  thin  quartzites  and  grayish  feldspathic  quartzites,  all  in  conformable  stratitica- 
tion,  as  a  whole.  Variously  interbedded  with  these  slates  and  quartzites,  from 
bottom  to  top,  are  beds  of  basic  eruptive  rock,  .     .     .  [pp.  47—48]. 

The  Animikie  series  of  Minnesota,  bearing  iron  at  one  horizon,  is  the 
equivalent  of  the  Marquette  series,  the  iron-bearing  group  of  Rominger,  of 
the  Peuokee-Gogebic  series  of  Michigan  and  Wisconsin,  of  the  Mesabi 
series  in  Minnesota,  of  the  Black  River  iron-bearing-  schists  in  Wisconsin, 
and  of  the  quartzites  of  the  Black  Hills.  All  are  of  Taconic  age,  for  the 
Lower  Cambrian  is  equal  to  the  Taconic,  the  Huronian  is  equal  to  the 
Taconic,  therefore  the  Lower  Cambrian  is  equal  to  the  Huronian  (pp.  46-48). 

La  the  Potsdam  sandstone,  which  is  uncomformably  on  the  Taconic, 
are  included  the  upper  quartzites  of  the  original  Huronian,  certain  of  the 
quartzites  of  Marquette,  the  Sioux  quartzites  of  Dakota,  and  the  quartzites 
of  Minnesota  and  Wisconsin.  This  is  also  the  age  of  the  copper-bearing 
rocks,  which  are  an  alternation  of  basic  and  acid  eruptions  with  interbedded 
sandstones  and  conglomerates.  The  great  gabbro  eruption  is  later  than  the 
beginning  of  the  Potsdam  age.  Unconformably  above  the  Potsdam  is  the 
St.  Croix  sandstone  (pp.  51-57). 

The  general  succession  in  descending  order,  is  as  follows  (p.  68): 

Calciferous.     Magnesian  limestones  and  sandstones..  | Dikelocephalus  horizon 

bt.  Croix,     bandstones  and  shales )  '^ 

Overlap  unconformity. 

Potsdam.     Quartzite,  gabbro,  red  granite,  and  Keweenawan_  __Paradoxides  horizon 

Ovet^laj)  unconformity. 

Taconic.     Black  and  gray  slates  and  quartzites,  iron  ore  (Huronian, 

Animike) .  _ - Olenellus  horizon 

Overlap  unconforin  Ity. 

Kewatin.     (Including  the  Kawishiwin  or  greenstone  belt,  with  its  jaspilite), 

sericitic  schists  and  gray  wackes -   

Vermilion.     (Coutchiching)  crystalline  schists 


Eruptire  uncorfortn ity. 
Laurentian.     (xneiss  — 


Archean 


EESUME  OF  LITERATURE.  93 

WiNCHELL,  H.  V.  Report  of  field  observations  made  during  the  season  of  1888 
in  the  iron  regions  of  Minnesota:  Seventeenth  Ann.  Rept.  Geol.  and  Nat.  Hist. 
Survey'  of  Minn.,  for  1888.  1889,  pp.  77-14.5;  see  also  The  diabasic  schists  containing 
the  jaspilite  beds  of  northeastern  Minnesota:  Am.  Geologist,  Vol.  HI,  1889,  pp.  18-22. 

In  the  above  H.  V.  Winchell  gives  further  observations  on  the  iron 
regions  of  Minnesota.  On  the  Giants  range  the  Animikie  is  found  to  rest 
upon  the  syenite.  There  is  a  semicrystaUine  rock  betvreen  tlie  two,  which 
gi-ades  into  the  syenite.  The  character  of  the  transition  is  not  metamorphic, 
but  rather  fragmental,  there  appearing  to  be  a  certain  amount  of  loose 
crystalline  material  which  has  resulted  from  the  decay  and  erosion  of  the 
syenite  lying  on  top  of  this  rock  in  the  bed  of  the  sea,  upon  and  around 
which  the  Animikie  sediments  were  deposited.  The  coarse  detritus  grades 
up  into  the  fine  detritus  of  the  Animikie  (p.  86).  The  Animikie  beds  are 
found  also  to  rest  unconformably  upon  the  upturned  edges  of  the  Keewatin 
schists  (p.  87).  The  same  relations  are  found  to  prevail  in  the  Birch  Lake 
region  (p.  91).  The  gabbro  containing  ores  in  the  vicinity  of  Kawishiwi 
River  are  found  to  contain  fragments  of  the  Animikie  slates  and  quartzites, 
and  is  therefore  of  later  origin  (pp.  96-97).  At  Gunflint  Lake  the  Animikie 
rests  unconformably  upon  the  Keewatin  (p.  104).  The  Keewatin  schists 
are  largely  of  eruptive  origin  (p.  132).  The  contacts  of  the  jaspilite  with 
the  basic  schists  are  abrupt  and  angular,  and  numerous  fragments  are  found 
contained  in  the  schists.  The  jaspilite  is  regarded  as  a  sedimentary  forma- 
tion, which  was  broken  up  and  involved  in  the  eruptions  of  Keewatin 
time.  The  Huronian  quartzite,  associated  with  the  magnetite,  lying 
unconformably  upon  the  syenite,  is  believed  to  lie  conformably  upon  the 
Animikie  slates  (p.  133). 

Grant,  Ulysses  S.  Report  of  geological  observations  made  in  northeastern 
Minnesota  during  the  summer  of  1888:  Seventeenth  Ann.  Rept.  Geol.  and  Nat.  Hist. 
Survey  Minn.,  for  1888, 1889,  pp.  147-215. 

In  this  report  Grant  gives  an  account  of  the  geologic  observations 
made  by  him  in  northeastern  Minnesota. 

North  of  Gunflint  Lake  the  vertical  Keewatin  slates  and  Vermilion 
crystalline  schists,  with  an  east  and  west  strike,  strike  directly  across  a 
range  of  immediately  adjacent  gneisses,  the  schists  showing  no  evidence  of 
being  twisted  or  bent  within  200  feet  of  the  gneiss.  In  the  syenites  of 
Gunflint  Lake  are  found  fragments  of  schist,  which  indicate  that  the  syenite 
is  eruptive  later  than  the  schists  (p.  159). 


94  THE  VERMILION  IRON-BEARING  DISTRICT. 

WiN'CHELL,  Alex.  Conglomerates  enclosed  in  gneissic  ten-anes:  Am.  Geologist, 
Vol.  III.  1889,  pp.  153-1*35.  256-262. 

Ill  this  jjaper  Alexander  Wiiicliell  restates  his  conclusions  concerning  the 
origin  of  the  Sagaiiaga  (Sixteenth  Ann.  Rept.,  p.  219)  and  Sea-Gull  (Sixteenth 
Ann.  Kept.,  p.  298)  syenite  conglomerate.  He  maintains  in  this  paper  that 
the  conglomerate  is  produced  from  a  fragmental  rock  by  selective  meta- 
morphism,  the  completely  crystalline  gneissoid  rocks  retaining  rounded 
frao-ments  which  are  residual  clastic  material.  The  conglomerate  of  Wonder 
Island  is  not  one  consisting  originally  of  a  mass  of  pebbles,  over  which  a 
fluid  magma  has  been  poured,  for  the  pebbles  are  not  in  contact;  they 
could  not  have  lain  where  they  are  before  the  magma  existed.  The  gneissic 
magma  was  contemporaneous  with  the  pebbles,  and  supported  them  and 
prevented  their  contact.  The  magma  must  have  been  plastic,  but  it  was 
low-temperature  igneo-aqueous  plasticity. 

^VI^'CHELL,  N.  H.  Some  thoughts  on  eruptive  rocks,  with  special  reference  to 
those  of  Minnesota:  Proc.  Am.  Assoc.  Adv.  Sci.,  Thirty-seventh  Meeting,  1889,  pp. 
212-221. 

N.  H.  Winchell,  in  1889,  in  a  general  discussion  of  the  origin  of  the 
eruptive  rocks,  maintains  that  there  are  four  epochs  of  basic  eruption  in 
Minnesota:  First,  that  represented  by  the  crystalline  schists  of  the  Vermilion 
group ;  second,  an  epoch  succeeding  the  gray wackes  of  the  Keewatin  and 
forming  a  part  of  the  Keewatin;  third,  one  succeeding  the  Animikie,  during 
which  the  Great  gabbro  or  Mesabi  overflow  was  outpoured. 

1890. 

Report  of  the  Royal  Commission  on  the  Mineral  Resources  of  Ontario  and 
Measures  for  their  Development.     Toronto,  1890,  pp.  123-126. 

In  this  report  there  is  a  brief  description  of  observations  made  by  three 
members  of  the  commission  who  visited  the  Vermilion  district.  No  state- 
ments of  geologic  character  which  are  of  any  interest  are  given.  State- 
ments were  gathered  from  miners  and  prosj^'^ctors  which  show  that  promising 
iron-bearing  formations  exist  in  Ontario  in  tlie  noi'theastern  extension  of  the 
Vermilion  iron  range. 

WixciiELL,  N.  H.  and  H.  V.  On  a  possible  chemical  origin  of  the  iron  ores 
of  the  Keewatin  in  Minnesota:  Am.  Geologist,  Vol.  IV,  1890,  pp.  291-300,  382-386; 
also  Proc.  Am.  Assoc.  Adv.  Sci.,  Thirty-eighth  Meeting,  1S90.  pp.  235-242. 


KESUME  OF  LITERATURE.  95 

lu  the  above  papers  N.  H.  and  H.  V.  Winchell  maintain  that  the  iron 
ores  of  the  Keewatin  of  Minnesota  are  not  derived  from  a  carbonate,  but 
are  probably  a  direct  chemical  precipitate;  for  there  is  no  evidence  of  the 
existence  of  carbonate  of  iron  at  any  time,  and  the  nature  of  the  country 
rock  is  such  as  to  imply  that  no  carbonates  in  amounts  required  could  he^ve 
been  deposited  at  the  time  the  rocks  were  formed. 

Winchell,  Alexander.  Some  results  of  ArchEean  studies:  Bull.  Geol.  See. 
Am..  Vol.  I,  1890,  pp.  357-394. 

Alexander  Winchell  in  1890  repeats  his  general  conclusions  as  to  the 
stratigra])hy  in  northeastern  Minnesota  already  given  in  his  reports  of  field 
work  in  the  Vermilion  district  for  the  yeai's  1886  and  1887,  and  published 
in  the  fifteenth  and  sixteenth  annual  reports  of  the  Minnesota  survey,  pages 
5-207  and  pages  133-391. 

Summed  up,  these  conclusions  are  briefly  as  follows:  In  northeastern 
Minnesota  there  are  large  areas  (in  the  Vermilion  district  four)  of  granitoid 
and  gneissoid  rocks  which  have  oval  outlines  trending-  in  general  northeast- 
southwest.  Gneissoid  rocks  predominate,  and  the  rocks  approaching  a 
granitoid  condition  are  foimd  only  at  the  centers  of  the  areas.  These 
gneissoid  areas  are  surrounded  by  cry  stalline  schists,  mica-schists,  hornblende- 
schists,  or  mica-hornblende-schists,  known  as  the  Vermilion  series.  These 
strike  east-northeast.  The  dip  increases  away  from  the  granitoid  areas 
until  it  becomes  vertical.  The  gneissoid  (granitoid)  rocks  and  schists  are 
intimately  connected,  and  the  author,  while  declining  to  make  a  definite 
statement,  clearly  intimates  that  the  gneiss,  g-ranites,  and  schists  are  all  of 
sedimentary  origin.     Referring  to  those  rocks,  he  says: 

They  are  so  inseparable  on  an}'  fundamental  g-rounds,  and  are  so  blended  together, 
both  structurally  and  mineralogically,  that  no  reasons  appear  to  exist  for  a  reference 
of  one  class  to  a  mode  of  origin  fundamentallj'  different  from  the  mode  of  origin  of 
the  other  class.  On  this  question,  however,  I  only  jDropose  at  present  to  cite  some 
observed  facts.     The  interpretation  of  them  maj^  be  subsequentlj-  undertaken. 

The  crystalline  schists  are  succeeded  bv  a  system  of  semicrystalline  schists  [the 
Keewatin].  Thej'  range,  however,  from  fragmentai  crj^stalline  to  earthv.  They, 
succeed  in  perfect  structural  conformity  with  the  older  schists,  with  only  slight  indi- 
cations of  stratigraphic  disturbance.  Their  attitude  is  generally  vertical  or  steeply 
inclined.     Their  position  is  between  and  surrounding  the  gneissoid  areas  [p.  377]. 

In  the  Vermilion  district,  as  they  lie  between  two  elongated  gneissoid 
areas,  they  have  a  persistent  east-northeast  strike  for  70  miles.     In  each  of 


96  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  intervals  between  such  gneissoid  areas  the  semicrystalline  schists  prevent 
the  structiire  of  a  simple  synclinal  fold.  Petrographically  the  semicrys- 
talline schists  (Keewatin)  are  sericitic  schists  inclosing  beds  of  hematite, 
argillite,  including  the  Ogishke-Muucie  conglomerate,  porphyrellite  and 
chloritic  schists,  porodites,  agglomerates  and  tuffs,  and  graywackes. 

Wherever  the  crystaUiue  and  semicry.staUine  schists  are  seen  in  juxtaposition 
their  stratification  is  strictly  conformable.  Whei'ever  the  crystalline  schists  are 
wanting  the  semicrystalline  schists  are  found  in  conformitA-  with  the  gneisses. 
Moreover,  whether  the  semicrystalline  schists  occur  in  juxtaposition  with  the  crys- 
talline schists  or  the  gneisses,  there  exist  frequently  those  transitions  by  alternation 
which  characterize  the  passage  from  the  crystalline  schists  to  the  gneisses.  This 
mode  of  transition,  however,  is  much  the  most  characteristic  of  the  passage  fj'om 
the  semicrystallines  to  the  crystallines  [p.  383]. 

Statement  is  also  made  of  the  petrographic  gradation  between  the 
semicrystalline  and  crystalline  schists. 

The  uncrystalline  schists  (Animikie)  are  chiefly  thin-bedded  black 
argillites  grading  into  graywackes  and  even  into  conglomerates,  with  flint 
and  jasper  schist  and  beds  of  magnetite.  These  rocks  overlie  the  semi- 
crystalline  Keewatin  schists  with  strong  unconformity. 

The  enumeration  made  embraces  all  rocks  up  to  the  Keweenawan. 
So  far  as  these  groups  are  concerned  the  order,  in  descending  succession,  is 
as  follows: 

5.  The  uncrystalline  schists  (Animikie,  Huronian). 

4.  The  semicrystalline  schists  (Keewatin). 

3.  The  crystalline  schists  (Vermilion). 

2.  The  gneissoid  rocks  1  ,_  , .     , 

,„,  .     . ,        ,     ;-  (Laurentian). 

1.   I  he  granitoid  rocks  J 

WiNCHELL,  N.  H.  and  H.  V.  The  Taconic  iron  ores  of  Minnesota  and  of 
western  New  England:  Am.  Geologist,  Vol.  VI,  1890,  pp.  263-274:. 

In  1890  N.  H.  and  H.  V.  Winchell  state  that  the  iron  ores  of  Minne- 
sota are,  at  five  different  geologic  horizons,  in  descending  order,  as  follows: 
(1)  The  hematites  and  liinonites  of  the  Mesabi  range,  the  equivalents  of 
the  hematites  of  the  Penokee-Gogebic  range  in  Wisconsin;  (2)  the  gabbro 
titaniferous  magnetites  near  the  bottom  of  the  rocks  of  the  Mesabi  range; 
(3)  olivinitic  magnetites,  just  below  the  gabbro  in  the  liasal  portion  of  the 
Mesabi  rocks;   (4)  the  hematites  and    magnetites  of  the  Vermilion  range 


\ 


RESUME  OF  LITERATURE.  97 

in  the  Keewatin  formation;  (5)  the  magnetites  of  the  crystalline  schists  of 
the  Vermilion  formation.  It  is  maintained  that  the  upper  iron  deposits 
of  the  Mesabi  and  those  of  the  Penokee-Gogebic  are  the  equivalents  of  the 
Taconic  ores  of  western  New  England. 

Irving,  R.  D.,  and  Van  Hise,  C.  R.  The  Penokee  iron-bearing  series  of  Michi- 
gan and  Wisconsin:  Mon.  U.  S.  Geol.  Survey  Vol.  XIX,  1892,  pp.  53i,  with  pis. 
and  maps.  See  also  Tenth  Ann.  Rept.  U.  S.  Geol.  Survey,  1890,  pp.  341-507,  with 
23  pis.  and  maps. 

Irving  and  Van  Hise  in  1890  and  1892  give  a  detailed  desci;iption  of 
the  Penokee  series  of  Michigan  and  Wisconsin,  and  of  the  complex  of  rocks 
south  of  this  series.  They  discuss  the  relations  which  the  Penokee  rocks  bear 
to  the  underlying  and  overlying  series,  as  well  as  to  tlie  Eastern  sandstone. 
The  Marquette  and  Felch  Mountain  series  of  Michigan,  the  Menominee 
series  of  Michigan  and  Wisconsin,  and  the  Animikie  and  Vermilion  Lake 
series  of  northeastern  Minnesota,  and  Ontario  are  alluded  to,  since  they  con- 
tain large  developments  of  rocks  which  are  almost  exact  reproductions  of  the 
iron  formation  rocks  in  the  Penokee  series.  The  Animikie  is  considered  in 
more  detail  than  the  rest,  since  a  comparison  of  its  iron-bearing  formation 
shows  that  it  consists  of  the  same  kinds  of  rocks  which  have  been  derived 
from  an  iron  carbonate  in  the  same  manner  as  those  of  the  iron  formation 
of  the  Penokee  series. 

A  further  comparison  of  the  Penokee  series  proper  and  the  Animikie 
series  shows  that  they  also  occupy  the  same  relative  positions  with  refer- 
ence to  overlying  and  underlving  rocks,  one  dipping  northward  under  the 
basin  of  Lake  Superior  and  the  other  dipping  southward  under  the  same 
body  of  water.  They  are  therefore  regarded  as  equivalent.  The  rocks  in 
various  other  areas  in  the  Lake  Supeiior  basin  refeiTcd  to  the  Upper 
Huronian  are  regarded  as  probably  equivalent  with  the  Penokee  series 

1891. 

Van  Hise,  C.  R.  An  attempt  to  harmonize  some  apparently  conflicting  views 
of  Lake  Superior  stratigraphy:  Am.  Jour.  Sci.,  3d  series,  Vol.  XLI,  1891,  pp. 
117-137. 

In  the  above  paper  Van  Hise  describes  the  physical  break  between  a 
Lower  and  an  Upper  Huronian  series.     That  the  two  series  are  separated 
MON  XLV — 03 7 


98  THE  VERMILION  IRON-BEARING  DISTRICT. 

by  a  great  unconformity  is  shown  by  numerous  contacts.  At  these  contacts 
the  lower  quartzite  of  the  Upper  series  contains  abundant  fragments  of  the 
Lower  series  which  had  reached  their  present  condition  before  being 
deposited  in  the  former.  That  the  Lower  series  has  been  greatly  folded  and 
deeply  truncated  before  the  Upper  series  was  deposited  is  further  shown 
by  the  much  banded  and  contorted  jasper  abutting  at  all  angles  against  the 
beds  of  theriptilted  but  simply  folded  Upper  series,  and  also  by  its  more 
crystalline  character. 

Since  great  belts  of  conglomerates  containing  abundant  fragments  of 
ore  and  jasper  are  found  in  the  Upper  Vermilion,  at  Ogisliki  Lake,  and  in 
the  Upper  Kaministiquia  series,  it  is  argued  that  the  source  of  this  material 
is  the  great  belts  of  iron  ore  and  jasper  contained  in  the  Lower  Vermilion, 
Hunters  Island,  and  Lower  Kaministiquia  series.  That  the  Vermilion  Lake 
conglomerate  is  unconformably  above  the  schists  in  vertical  attitude, 
bearing  ore  and  jasper,  is  further  indicated  by  the  fact,  discovered  by 
Merriam,  that  on  the  islands  of  Vermilion  Lake  the  conglomerate  is  found 
to  be  in  a  series  of  gentle  folds  although  having  a  vertical  cleavage 
developed.  Merriam  regards  the  conglomerate  as  a  comparatively  thin 
formation  overlying  and  overlapping  the  Lower  series.  Both  the  Animikie 
and  Upper  Vermilion  are  unquestionably  separated  from  the  Lower  Ver- 
milion by  an  unconformity.  Tlie  Animikie  is  believed  to  be  the  equivalent 
of  the  Upper  Vermilion. 

It  is  concluded  that  the  confusion  in  correlation  of  the  formations  about 
Lake  Superior  is  due  to  the  failure  to  recognize  this  general  unconformity. 
Once  recognized,  the  structural  conclusions  to  which  the  various  writers 
have  most  steadfastly  held  are  found  to  be  in  general  harmony.  Above 
the  physical  break,  and  constituting  the  Upper  Huronian  (equivalent  to  the 
Original  Huronian)  are  the  Animikie  and  the  Upper  Kaministiquia,  Upper 
Vermilion,  Upper  Marquette,  Western  Menominee,  Penokee-Gogebic  proper, 
the  Dakota,  Iowa,  Minnesota,  and  Wisconsin  quartzites  surrounded  by  the 
fossiliferous  series.  In  the  Lower  Huronian  is  the  Kewatin  (in  part  at 
least),  the  Lower  Kaministiquia,  Lower  Vermilion,  Lower  Marquette,  Felch 
Mountain  iron-bearing  series,  Menominee  ]n-oper,  and  the  Cherty  limestone 
at  the  base  of  the  Penokee  series,  and  the  Black  River  Falls  iron-beai-ing 
schists. 


RESUME  OF  LITERATURE.  99 

Lawson,  Andrew  C.  Lake  Superior  stratigraphy:  Am.  Geologist,  Vol.  VII, 
1891,  pp.  320-327. 

Lawson  discusses  Lake  Superior  stratigraphy  in  an  article  which 
owes  its  inception  to  the  paper  by  Prof.  C.  R.  Van  Hise  entitled  "An 
attempt  to  harmonize  some  apparently  conflicting  views  of  Lake  Superior 
stratigraphy,""  which  is  abstracted  above,  p.  97. 

Lawson  argues  for  the  indivisibility  of  the  Archean,  meaning-  by  this 
term  all  those  rocks  that  existed  prior  to  the  denudation  epoch,  during 
which  time  the  floor  was  formed  on  which  has  since  been  deposited 
with  strong  unconformity  the  Animikie  series  and  its  equivalents.  The 
Archean  thus  includes,  in  upward  succession,  the  Laurentian  gneiss  and 
granite,  the  crystalline  schists  of  the  Coutchiching,  and  the  crystalline 
schist  and  elastics  of  the  Keewatin.  The  Coutchichincf  and  Keewatin  are 
so  knit  together  by  the  Laurentian  foliated  granite  as  to  warrant  the  union 
of  all  of  these  under  the  term  Archean. 

Lawson  then  argues  against  Van  Hise's  correlation  of  the  Upper  Ver- 
milion with  the  Animikie  series.  He  states  tliat  the  granite  of  Sag'anaga 
Lake  is  found,  with  abundant  and  clearly  observed  evidences  of  eruption, 
breaking  through  the  Keewatin  rocks,  including'  the  Upper  Vermilion  (Van 
Hise)  fragmental  rocks  of  Ogishki  Lake  with  their  associated  slates  and 
grits.  It  is  concluded  that  the  break  between  the  Upioer  and  the  Lower 
Vermilion  described  by  Van  Hise  is  within  the  Keewatin  group,  dividing 
it  into  an  upper  and  a  lower  series,  and  that  this  break  is  therefore  below 
the  Animikie;  that  these  series  are  united  by  the  Saganaga  graiiite  intru- 
sions and  that  they  belong  in  the  schist-granite-gneiss  complex  of  the 
Archean;  hence  this  break  is  below  the  Animikie.  It  is  further  said  that 
the  conglomerates  of  the  Upper  Kaministiquia  series  come  out  close  to  the 
shores  of  Thunder  Bay  and  form  the  basement  upon  which  tlie  undisturbed 
Animikie  rock  rests  with  strongly  marked  unconformity.  The  following 
succession  foi-  the  region  northwest  of  Lake  Superior  is  presented: 


[Keweenawaii,  or  Nipigou  group. 
Paleozoic — Algonkian  system]  Unconformity. 

[Animikie  group  (possibl}'  Huronian) 

a  Am.  Jour.  Sci.,  3d  series,  Vol.  XLI,  1891,  pp.  117-1:^7. 


100  THE  VERMILION  IRON-BEARING  DISTRICT. 


Unconformit)'.     Greatest  erosion  interval  in  American  geology. 


Keewatin  group  (possibly  Huronian) 


Archean 


Ontarian  systemjuueonformity  (?). 

Coutchiching  group. 
Irruptive  unconformity. 
Laurentian  s  system. 


Upper  series. 
Van  Rise's  break. 
Lower  series. 


WiNCHELL,  N.  H.  Record  of  field  observations  in  1888  and  1889:  Eighteenth 
Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  of  Minn.,  for  1889,  1891,  pp.  7-47. 

N.  H.  Wiuchell  in  1891  gives  numerous  additional  field  observations. 
The  relations  of  tlie  jaspilite,  argillite,  and  green  schist  are  considered,  and 
the  argillite  at  least  is  regarded  as  a  sedimentary  rock  (p.  9).  In  the  Stuntz 
conglomerate  is  found  a  large  bowlder  which  contains  pebbles  of  chalce- 
donic  quartz  and  quartzose  felsite — these  contain  pebbles  of  vitreous 
quartz — and  pebbles  resembling  the  porphyrel  at  Kekekebic  [Cacaquabic] 
Lake  (p.  31).     A  study  of  the  ore  formation  leads  to  the  conclusion  that — 

All  three  of  the  known  agencies  for  rock  forming  were  intermittent!}^  at  work 
and  concerned  in  the  formation  of  the  iron  ore,  viz:  Eruption  to  afford  the  basic 
eruptive  material;  sedimentation,  to  arrange  it  (in  the  main),  and  chemical  precipita- 
tion in  the  same  water,  to  give  the  pure  hematite  and  chalcedonic  silica  [p.  42]. 

The  following  facts  are  given  as  evidence  that  the  Grreat  gabbro  of  the 
Cupriferous  formation  lies  below  the  Animikie  slates  and  that  the  Kewee- 
nawan  includes  both  the  Animikie  slate  and  the  Huronian  (Potsdam) 
quartzite. 

The  most  important  and  significant  fact  that  bears  on  the  stratigraphical  position 
of  the  gabbro,  respecting  its  relation  to  the  Animike  black  slates,  is  its  occurrence 
along  a  wide  extent,  reaching  from  Gunfiint  Lake  as  far  southwestward  as  to  the 
railroad  crossing  at  Mallmann's  (at  least),  next  to  and  immediately  south  either  of  the 
gneiss  of  the  Giant's  range  or  of  the  "greenstones"  of  the  Kawishiwin,  without  the 
appearance  of  any  of  the  lilack  slates  between  them.  There  is  an  appearance  of 
quartzite,  with  olivine  grains  and  with  magnitite,  geographically  between  the  gneiss 
and  the  gabbro,  the  same  being  unquestionably  the  Pewabic  quartzite  seen  near 
Gunflint  Lake.  This  quartzite  is  sometimes  impure  and  linionitic,  and  seems  to  be 
the  chief  iron  horizon  of  the  Mesabi  range.  This  near  conjunction  (which  is  some- 
times apparently  an  exact  contact)  of  the  gabbro  with  the  gneiss,  and  the  absence  of 
the  Animikie  proper  between  them,  has  been  supposed  to  be  due  to  a  local  overlap 
of  the  gabbro  beyond  the  strike  of  the  Animike,  covering  it  from  sight,  the  idea 


RESUME  OF  LITERATURE.  101 

being  that  the  gabbro  flowed  back  noithward  over  older  formations  and  came  onto 
the  gneiss  [pp.  -itt-iS]. 

Bowlders  of  characteristic  gabbro  and  red  syenite,  and  of  quartz  porph}- ry,  occur 
abundantly  in  the  later  "traps"  of  the  Cupriferous  [p.  i5]. 

WiNCHELL,  H.  V.  Geological  age  of  the  Saganaga  syenite:  Am.  Jour.  Sci.,  3d 
series,  Vol.  XLI,  1891,  386-390. 

H.  V.  Winchell  in  1891  states  that  the  syenite  of  Sagauaga  Lake  is 
conglomeratic  in  places  and  contains  pebbles  which  are  similar  to  each 
other,  being  mostly  composed  of  lamellar  angite,  with  or  without  grains  of 
feldspar,  but  there  are  no  pebbles  of  sedimentary  rocks  or  of  syenite  or 
jasper  such  as  occur  in  the  Kewatin  conglomerates. 

In  the  Saganaga  syenite  at  the  end  of  the  portage  on  Granite  River 
is  a  band  of  silica  1^  inches  in  diameter  and  3  feet  in  length.  This  is 
presumed  to  have  been  formed  by  chemical  j^i'ecipitation  from  heated 
oceanic  waters."* 

North  of  Saganaga  Lake  the  syenite  grades  into  greenish  feldspathic 
and  sericitic  schists  and  agglomerates  without  the  usual  intervening  belt  of 
crystalline  Vermilion  schists.  From  these  facts  it  is  concluded  that  the 
syenite  is  simply  the  result  of  the  locally  intense  metamorphism  of 
Kewatin  rocks,  and  is  thus  of  Kewatin  age. 

Finally,  as  bearing  upon  the  economic  side  of  the  question  it  is 
suggested  that — 

If  the  Saganaga  syenite  be  of  the  Keewatin  age  and  contain  chalcedonic  silica  in 
an  original,  unchanged  condition  it  is  not  unlikely  to  contain  also  Keewatin  iron-ore 
deposits  free  from  titanium  and  of  high  grade  in  other  respects.  It  can  thus  no 
longer  be  laid  down  as  a  law  for  explorers  in  the  Northwest  that  the  gneisses  contain 
no  iron-ore  deposits  [p.  390]. 

AViNCHELL,  N.  H.  and  H.  V.,  The  iron  ores  of  Minnesota,  Bull.  No.  6,  Minn. 
Geol.  and  Nat.  Hist.  Survej',  1891,  430  pages,  with  geological  map  and  section. 

.  N.  H.  and  H.  V.  Winchell  in  1891  give  an  extended  treatment  of  the  iron 
ores  of  northeastern  Minnesota  and  the,  rocks  in  which  they  are  contained. 
Magnetic  iron  ore  is  not  of  great  importance.  Isolated  deposits  are  reported 
in  the  mica-hornblende-schists  and  in  a  massive  hornblende-mica  rock  of 

«  Winchell,  N.  H.  and  H.  V.,  On  a  possible  chemical  origin  of  the  iron  ores  of  the  Keewatin  in 
Minnesota:  Proc.  Am.  Assoc.  Adv.  Sci.,  Thirt3'-eighth  Meeting,  1890,  pp.  235-242.  Am.  Geologist, 
Vol.  IV,  1891,  pp.  291-300;  382-386.  Also  The  iron  ores  of  Minn.,  Geol.  iSfat.  Hist.  Survey  ilinu., 
1889,  Bull.  No.  6,  430  pages. 


102  THE  VERMILION  IKON-BEARING  DISTRICT. 

the  Vennilion  series.  Deposits  in  the  schists  are  presumed  to  be  of  sedi- 
mentary origin,  the  magnetite  having  been  produced  by  hydrothermal 
action  £i-om  hematite,  Hke  that  of  the  Keewatiu.  Those  magnetite.s  in  the 
massive  rocks  are  of  igneous  origin  and  are  analogous  to  the  titaniferous 
magnetite  found  in  the  gabbro. 

Hematite  is  the  only  ore  actually  rained  on  the  Vermilion  range  up  to 
tliis  time,  and  therefore  the  deposits  of  iron  of  this  character  are  the  most 
important  and  the  ones  with  which  this  report  chiefly  deals. 

The  ore  is  always  found  in  schistose  or  massive  greenstone  of  Keewatin 
age  and  is  -always  associated  with  jaspilite.  The  ore  bodies  vary  much  in 
size  and  are  of  lenticular  shape,  with  long  axis  trending  southwest-northeast, 
parallel  to  the  schistosity  of  the  inclosing  schists. 

The  jaspihte  and  schist  of  the  Keewatin  are  found  to  occur  sometimes 
minutely  interlaminated ;  at  other  times  the  jasper  is  in  irregular  layers, 
which  never  have  any  great  extent,  and  finally  always  pincli  out;  at  other 
times  it  is  in  oval  forms,  the  greater  lengths  being  parallel  with  the  schistose 
structure.  Again,  the  jaspihte  is  in  great  fragments  within  the  green  or 
massive  diabasic  schists,  the  masses  having  sometimes  such  relations  with 
each  other  as  to  show  that  they  are  a  broken  continuous  layer.  The 
branches  from  the  large  bodies  of  jaspilite  are  supposed  to  be  Caused  by 
the  crumpling,  breaking,  and  squeezing  of  the  entire  rock  structure,  by 
which  the  thinner  sheets  have  been  buckled  out  and  thrust  laterally  among 
the  inclosing  schists.  The  ore  and  jasper  are  regarded  as  a  direct  chemical 
deep-sea  precipitate  from  an  ocean  of  hot  alkaliuic  water  which  was 
continually  disturbed  by  acid  rains  and  flows  of  basic  lava  due  to  volcanic 
activity.     The  iron  for  the  ore  was  extracted  from  the  basic  lavas. 

The  rocks  of  the  Animikie,  equivalent  to  the  Huronian  and  included  in 
the  Taconic,  consist  chiefly  of  carbonaceous  and  argillaceous  slates,  with 
siliceous  slates,  fine-grained  quartzites,  and  gray  limestones.  At  the  bottom 
of  the  series  is  a  fragniental  quartz  sandstone,  300  feet  in  thickness,  whicli  is 
named  the  Pewabie  quartzite.  The  slates,  conglomerates,  and  quartzites  are 
profoundly  affected  and  intermingled  with  eruptive  material  similar  to  that 
found  so  abundantly  in  the  Kewatin.  These  beds  have  the  appearance  of 
consolidated  beds  of  basic  lava  or  of  porous  tuff,  but  where  this  prevails 
there  is  a  sensible  gradation  from  the  dark  trap-looking  beds  to  the  thin 
beds  of  slate.     At   Ogishke  Muncie  Lake  there   is  a   slate   conglomerate 


o 


RESUME  OF  LITERATURE.  103 

similar  to  that  on  the  north  shore  of  Lake  Huron.  This  conglomerate  is 
not  the  same  as  the  ag'glomerates  of  the  Kewatin,  such  as  that  on  Stuntz 
Island,  at  Vermilion  Lake,  and  Ely.  The  Kewatin  is  always  nearly  vertical, 
while  the  dip  of  the  Taconic  rarely  exceeds  15°.  The  iron-ore  beds  of  the 
Taconic  are:  (1)  The  quartzose,  hornblendic  (or  olivinitic),  magnetitic  group 
of  the  Pewabic  quartzite;  (2)  an  impure  jaspilite,  hematite,  and  limonite 
group;  (3)  a  carbonated  iron  group;  (4)  a  gabbro  titanic  iron  group.  The 
jaspilitic  hematite  group  has  the  same  lithologic  peculiarities  as  the 
jaspilite  beds  of  the  Vermilion  range.  The  gabbro  in  which  the  titanic 
iron  occurs  constitutes  the  Mesabi  range.  This  has  been  before  regarded 
as  the  base  of  the  Keweenawan,  into  which  it  fades  upwardly,  but  it  has  been 
found  that  this  great  gabbro  flow  was  outpoured  at  an  earlier  date,  and  it  is 
placed  at  or  near  the  bottom  of  the  Animikie. 

Considered  as  to  origin,  the  ores  of  the  Taconic  found  in  groups  1  and 
2  above  are  supposed  to  be  due  to  chemical  oceanic  precipitation.  Those 
of  group  4  are  of  ig'ueous  origin  and  are  an  integral  part  of  the  gabbro 
The  origin  of  those  of  group  3  is  not  definitely  stated  (pp.  144-145). 
None  of  these  Taconic  ores  are  thus  far  mined  in  Minnesota. 

1S93. 

Batlet,  W.  S.  Notes  on  the  petrography'  and  geology  of  the  Akeley  Lake 
region  in  northeastern  Minnesota:  Nineteenth  Ann.  Rept.  Geol.  and  Nat.  Hist. 
Survey  Minn.,  for  1890,  1893,  pp.  193-210. 

This  is  chiefly  a  petrographic  description  of  rocks  from  Akeley  Lake. 
The  three  important  results  reached,  largely  by  the  microscopic  study,  are 
summarized  as  follows : 

(1)  Most  of  the  rocks  designated  as  Pewabic  quartzite  in  the  neighborhood  of 
Akelej'  Lake  are  not  quartzites,  but  they  are  granulitic  phases  of  gabbro.  The 
remainder  are  crystaUized  aggregates  of  quartz.  None  of  them  are  sedimentary 
rocks,  and  consequently  none  can  serve  to  determine  the  age  of  the  ore  associated 
with  them  or  of  the  gabbro  in  which  they  occur. 

(i:)  On  the  other  hand,  the  granulitic  gabbros  may  be  traced  into  true  granitic 
gabbros  and  into  quartzose  phases  of  granulitic  varieties.  Hence,  the  granulitic 
beds  and  their  associated  ores  are  of  the  same  age  as  the  gabbro,  whose  structural 
relations  to  the  younger  and  older  formations  must  be  appealed  to  in  order  to  settle 
the  question  of  age. 

(3)  Since  so  much  of  the  "Pewabic  quartzite"  is  not  quartzite  in  anj'  sense  of 
the  word,  and  since  different  beds  that  have  been  given  this  name  are  not  all  certainly 


104  THE  VERMILION  IRON-BEARING  DISTRICT. 

of  the  same  age,  it  is  evident  that  great  care  must  be  taken  in  the  use  of  the  "  Pewabic 
quartzite"  for  correlation  purposes.  Several  different  rocks  have  been  included 
under  this  one  title,  hence  the  "Pewabic  quartzite"  as  defined  can  not  be  relied 
upon  as  marking  a  definite  horizon  in  the  succession  of  the  geological  formations  in 
northeastern  Minnesota  [pp.  208-209]. 

In  an  appendix  Dr.  Bayley  states  that  the  first  two  of  the  above 
conchisions  had  been  reached  by  W.  M.  Chauvenet  a  number  of  years 
before,  as  the  result  of  work  done  in  the  vicinity  of  Akeley  Lake  for  the 
Lake  Superior  division  of  the  United  States  Geological  Survey  in  1883  and 
1884.  Dr.  Bayley  was  not  aware  of  Mr.  Chauvenet's  conclusion's  until 
after  his  own  had  been  arrived  at,  Mr  Chauvenet's  being  contained  in  an 
unpublished  report  submitted  to  Prof  R.  D.  Irving  (p.  209). 

WiNCHELL,  N.  H.     The  Kawishiwin  agglomerate  at  Ely,  Minn. :  Am.  Geologist, 
Vol.  IX,  1892,  pp.  359-368. 

This  article  contains  a  description  of  a  greenstone  which  is  found  in 
the  Keewatin  of  the  Vermilion  district  of  Minnesota  and  which  possesses  a 
peculiar  structure. 

On  clean  exposures  the  greenstone  is  seen  to  be  not  homogeneously 
massive,  but  to  be  composed  of  irregularly  rounded  to  oval  bodies  of  mas- 
sive greenstone  ranging  from  6  to  15  inches  in  diameter,  surrounded  by 
and  separated  from  each  other  by  relatively  narrow  masses  of  fine-grained 
chloritic  schist,  which  winds  about  among  the  masses,  its  schistosity  coin- 
ciding with  the  surfaces  with  which  it  is  in  contact.  The  peripheries  of  these 
massive  bodies  are  all  amygdaloidal,  the  long  direction  of  the  pores  being 
perpendicular  to  the  surface  of  the  round  body. 

The  author  explains  the  rock  as  an  agglomerate,  the  rounded  bodies 
being  bombs  that  were  hurled  into  the  air  by  volcanic  forces  and  fell  into  a 
hot  ocean,  in  which  was  being  deposited  a  fine  volcanic  mud  which  now 
forms  the  fine  schist  between  the  bombs.  In  other  words,  "The  source  of 
such  rocks  was  igneous,  but  their  structure  is  aqueous"  (p.  367). 

Van  Hise,  C.   R.     Correlation  papers,   Archean  and  Algonkian:  Bull.   U.  S. 
Geol.  Survey  No.  86,  1892,  pp.  51-208,  map,  PI.  Ill,  op.  p.  52,  and  pp.  440-529. 

This  bulletin,  on  the  pages  indicated,  contains  a  thorough  digest  of  the 
various  articles  that  had  appeared  concerning  the  Lake  Superior  pre-Cam- 
briau  geology  prior  to  the  time  of  its  publication.  The  author's  interpretation 
of  the  structure  and  stratigraphy  of  the  various  regions  is  based  upon  his 


RfiSUME  OF  LITERATURE. 


105 


very  w'de  personal  kuowledg-e  of  the  occurrences  described  in  the  articles, 
and  this  lends  additional  value  to  his  opinion. 

The  following  are  his  conclusions  about  the  Vermilion  district,  with 
which  we  are  especially  interested.  The  succession,  compared  with  that  of 
the  Marquette  district  of  the  south  shore  of  Lake  Superior,  which,  having 
been  carefully  worked  out,  we  can  use  as  a  standard,  is  as  follows  (p.  195): 

Northern  Minnesota.  Marquette  (Michigan)  district. 

Keweenawan. 
Unconformity. 
Alffonkian  jAnimikie  and  Upper  Ver-  Upper  Marquette. 

milion. 
Unconformity. 
Lower  Vermilion. 
Unconformitj'  (?). 
(Coutchiching?). 
Eruptive  unconformity 


Unconformity. 
Lower  Marquette. 
Unconformity. 


Archean 


Fundamental  complex  (not  yet 
separated  in  mapping). 


Laurentian. 


Grant,  U.  S.     The  stratigraphic  position  of  the  Ogishke  conglomerate  of  north- 
eastern Minnesota:  Am.  Geologist,  Vol.  X,  1S92,  pp.  4-10. 

Grant  states  that  the  Animikie  rests  unconformably  upon  the  Saganaga 
granite;  that  the  Ogishke  conglomerate  is  intruded  by  the  Saganaga  granite, 
and  therefore  that  the  Ogishke  conglomerate  is  earlier  than  and  separated 
by  a  great  structural  break  from  the  Saganaga  granite.  As  the  Keewatin 
has  the  same  relations  to  the  Saganaga  gi'anite  as  the  Ogishke  conglomerate, 
the  same  thing  is  true  of  the  Animikie  and  Keewatin. 

The  Ogishke  conglomerate  is  younger  than  the  most  of  Keewatin,  but 
is  considered  as  a  part  of  it. 

1S93. 

Van  Hise,  C.  R.     An  historical  sketch  of  the  Lake  Superior  region  to  Cambrian 
time:  Jour.  Geol.,  Vol.  L  1893,  pp.  113-128. 

In  this  historical  sketch  the  five  subdivisions  given  for  this  region  are 
the  Basement  complex  or  Archean,  the  Lower  Huronian,  Upper  Huro- 
nian,  and  Keweenawan,  the  last  three  together  constituting  the  Algonkian 
and  the  Lake  Superior  (Cambrian)  sandstone.  Each  of  these  divisions  is 
separated  from  the  others  by  unconformities.  The  only  rocks  of  the  Ver- 
milion district  treated  of  are  the  Lower  Vei'milion  and  the  Animikie  series, 


106  THE  VERMILION  IRON-BEARINCx  DISTRICT. 

tlie  Lower  Vermilion  series  being  placed  in  the  Lower  Huronian  and  the 
Animikie  being  placed  in  the  Upper  Hnronian. 

The  Lower  Huronian  is  largely  crystalline;  the  L'pper  Huronian  semi- 
crystalline.  Locally,  along  axes  of  intense  plication,  both  the  Lower 
Huronian  and  Upper  Huronian  have  been  transformed  into  completely 
crystalline  schists. 

WixcHELL,  N.  H.  The  crystalline  rocks — some  preliminary  considerations  as 
to  their  .structure  and  origin:  Twentieth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey 
Minn.,  for  1891,  1893,  pp.  1-28. 

In  this  aiticle  Professor  Winchell  g^ives  the  following'  as  the  descend- 
ing  succession  of  strata  in  northeastern  Minnesota.  This  detemiination  of 
the  succession  represents,  according  to  him,  the  consensus  of  opinion  of 
several  geologists  who  have  given  special  attention  to  the  field  evidences. 
From  this  statement  must,  however,  be  excepted  the  Great  gabbro  horizon, 
No.  3,  as  by  some  it  is  presumed  to  have  preceded,  and  not  to  have 
followed,  the  Pewabic  quartzite. 

1.  Keweenawan  or  Nipigon  series,  unconformablv  beneath  rocks 
bearing'  the  "Dikellocephalus"  fauna,  and  consisting  of  fragmental  and 
eruptive  beds,  the  upper  portions  being  almost  entirely  red  sandstones. 

2.  Alternating  beds  of  eruptive  sheets  and  fragmental  rocks.  The 
fragmentals  are  thin-bedded  slates,  actinolite-schists,  magnetitic  jaspers, 
cherts,  and  quartzites.     The  sheets  are  ordinary  eruptives  or  pyroclastics. 

3  Immense  quantities  of  true  gabbro,  often  bearing  titaniferous  mag- 
netite, are  associated  with  contemporaneous  felsites,  quartz-porphvries, 
and  red  granites.  This  gabbro  includes  several  masses  of  the  next  older 
strata,  particularly  the  Pewabic  quartzite. 

4.  The  Animikie.  This  series  is  characterized  by  a  great  quartzite 
associated  with  the  iron  ores  and  cherts.  The  quartzite  (Pewabic)  lies 
iinconfomiably  on  all  the  older  rocks.  It  is  often  conglomeratic,  bearing 
debris  of  the  underlying  formations.  Within  it  is  mingled  volcanic  tufts 
from  contemporaneous  eruptions.  The  Pewabic  quartzite  includes  that  of 
Pokegama  Falls,  on  the  Mississippi,  and  of  Pipestone  County.  In  the 
vicinity  of  contemporaneous  volcanic  disturbances  its  grain  is  fine,  like 
jasper,  and  sometimes  it  has  acquired  a  dense  crystalline  sti'ucture  fi'om 
contact  with  the  gabbro. 

5.  The  Keewatin.  This  is  a  volcanic  series  of  great  thickness,  com- 
posed mainly  of  volcanic  tuffs,  presenting  evidence  of  aqueous  sedimen- 


EfiSUME  OF  LITERATURE.  107 

tation,  but  conglomerates,  graywackes,  quartzitic  schists,  and  glossy 
serpentinous  schists  are  present.  The  Kawishiwin  formation,  apparently 
the  upper  member  of  the  series,  embraces  the  great  bulk  of  the  greenstones, 
chloritic  schists,  jaspers,  and  hematites.  The  iron  ores  are  in  lenticular 
lodes,  and  stand  upright,  conformable  with  the  general  position  of  the  rocks. 

6.  The  Keewatin  series  becomes  more  crystalline  toward  the  bottom, 
and  passes  conformably  into  completely  crystalline  mica-schists  and  horn- 
blende-schists, which  are  named  the  Vermilion  series.  The  rocks  are  usu- 
ally stratiform,  contain  magnetic  iron  ore,  and  embrace  some  dark  massive 
greenstone  belts,  in  which  no  stratification  bands  are  visible. 

7.  The  Laurentian.  When  not  disturbed  by  upheaval  the  Vermilion 
schists  pass  into  Lain-entian  gneiss,  there  being  a  gradual  increase  in  the 
feldspathic  and  siliceous  ingredients.  Even  after  the  Laurentiaji  characters 
are  apparently  fully  established,  conformable  bands  of  Vermilion  schists 
reappear,  from  which  it  is  plain  that  the  base  of  the  Vermilion  is  an  uncer- 
tain plane,  which  can  not  be  located  exactly.  This  normal  passage  from 
the  Vermilion  to  the  Laurentian  is  frequently  disturbed  by  the  intrusion  of 
numerous  dikes  of  light-colored  granitic  and  basic  rocks.  These  were 
both  in  a  fluid  state,  the  only  nonfluid  rocks  being  the  schists  which  are 
embraced  within  them  in  isolated  pieces.  In  a  similar  manner  small 
areas  of  Laurentian  granite  are  sometimes  directly  in  contact  with  schists, 
which  have  the  imperfectly  crystalline  condition  of  the  Keewatin. 

Nos.  3  and  4  are  separable  from  No.  2  by  divergence  in  dip  and  strike, 
as  well  as  by  a  marked  diiference  of  lithology.  There  is  consequently  some 
evidence  of  unconformity  between  them.  Below  No.  4  is  a  great  physical 
break,  which  separates  Nos.  1,  2,  3,  and  4  from  5,  6,  and  7  throughout  the 
Lake  Siiperior  region.  This  break  is  the  greatest  erosion  interval  which 
has  been  discovered  in  Paleozoic  geology.  Nos.  1,  2,  3,  and  4  together 
constitute  the  Taconic.  Nos.  5,  6,  and  7  constitute  the  fundamental  com- 
plex or  Archean,  which  is  a  unit  in  its  grander  featui'es. 

Grant,  U.  S.  Field  observations  on  certain  g-ranitic  areas  in  northeastern  Minne- 
sota: Twentieth  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Minn.,  for  1891,  1893, 
pp.  35-110.     One  map. 

Grant,  in  1893,  publishes  his  notes  made  on  a  trip  in  northeastern 
Minnesota.  The  areas  visited  were  those  of  Kawishiwi  River,  Snowbank 
Lake,  Kekequabic  [Cacaquabic]  Lake,  and  Saganaga  Lake. 


108  THE  VERMILION  IRON-BEARING  DISTRICT. 

In  the  study  of  tliese  areas  there  was  uo  evidence  found  of  a  transition 
from  semicrystalline  and  crystalline  schists  into  granite.  On  the  other 
hand,  abundant  evidence  was  found  of  the  irruptive  nature  of  the  granite 
rocks  into  the  surrounding  sediments.  The  Kawishiwi  River  and  Snow- 
bank Lake  massive  rocks  are  hornblende-syenites.  The  Saganaga  rock  is 
a  coarse  hornblene-granite.  That  around  Kekequabic  Lake  is  a  pyroxene 
granite,  and  associated  with  it  is  peculiar  pyroxene-granite-porphyry 
(pp.  37-38). 

The  intrusive  character  of  the  granite  is  particularly  well  shown  where 
the  line  between  sees.  31  and  32,  T.  63  N.,  R.  10  W.,  cuts  the  shore  of 
Clearwater  Lake,  and  in  the  SE.  i  of  the  SW.  \  sec.  26,  T.  64  N.,  R.  9  ^Y., 
on  the  west  shore  of  Snowbank  Lake. 

[Along-]  the  Kawishiwi  River  .  .  .  iive  distinct  rock  tj^pes  [gabbro,  syenite, 
mica-schist,  graywacke,  etc.,  greenstone,  and  quartz-porphyrj-]  are  present.  The 
gabbro  is  the  most  recent;  it  covers  part  of  the  older  rocks.  .  .  .  The  .syenite  is 
older  than  the  gabbro  and  Is  younger  than  the  greenstone  and  mica-schist,  both 
of  which  it  cuts.  .  .  .  The  mica-schists,  graywackes,  etc.,  stand  vertical,  and  have 
a  general  east-northeast  stiike;  thej'  belong  to  what  has  been  mapped  as  the  Ver- 
milion series,  but  there  seems  to  be  good  reason  for  putting  all  of  this  type  of  rocks, 
in  the  area  of  this  map,  into  the  Keewatin.  The  greenstone  is  presumably  of 
Keewatin  age,  and  is  .probably  younger  than  the  mica-schists,  graywackes,  etc. 
Quartz-porphyrj'  dikes  are  found  cutting  the  greenstones  in  several  places,  but 
they  have  not  been  seen  in  the  other  rocks  in  this  immediate  vicinity  [p.  59]. 

The  conclusions  of  this  report  differ  from  the  general  succession  given 
by  Professor  AVinchell  in  the  fundamental  point  that  there  is  no  gradati&n 
between  the  granitic  rocks  and  the  metamorphosed  sedimentary  rocks. 
Also  all  of  the  metamorphosed  sedimentary  rocks  are  regarded  as  belonging 
to  the  Keewatin  (Lower  Huronian  '?),  while  the  Vermilion  schists  are  not 
found.  If  there  now  exists  in  this  area  the  original  basement  upon  which 
the  sedimentary  rocks  were  deposited,  this  has  not  been  found.  It  is  of 
course  possible  that  such  a  Basement  complex  does  not  exist  in  the  Kawishiwi 
River  area,  the  one  which  was  most  closely  studied. 

Grant,  U.  S.  The  geology  of  Kekcqualnc  Lake  in  northeastern  IMinnesota, 
with  special  reference  to  an  augite  soda-granite:  Twenty-tirst  Ann.  Rept.  Geol. 
and  Nat.  Hist.  Survey  Minn.,  for  1892,  1893,  pp.  5-58.     With  map,  PI.  II. 

In  tliis  article  Grant  describes  in  great  detail  an  area  approximately 
ilcs  s([uai-e   surnninding  Kekequabic  Lake.     The  distribution  of  the 


RESUME  OF  LITERATURE.  109 

various  kinds  of  rocks  present  in  the  area  is  carefully  determined  and 
represented  upon  the  map  (PL  II),  which  accompanies  the  article.  With 
the  exception  of  the  Keweenawan  gabbro  and  certain  diabase  dikes,  whose 
age  is  undetermined,  all  the  rocks  described  are  iiicluded  in  the  Keewatin. 
The  points  of  chief  interest  in  the  paper  are  of  a  petrographic  character, 
and  consist  in  a  description  of  some  anomalous  green  schists,  of  a 
hornblende-porphyrite,  and  of  an  augite  soda-granite.  Evidence  is  also 
presented  to  show  that  this  is  a  true  igneous  granite,  and  is  not  due  to 
crystallization  of  sediments  in  situ,  as  had  been  previously  maintained  in 
papers  on  the  region  hj  other  writers. 

1894. 

Grant,  U.  S.  Preliminary  report  of  tield  work  during  1893  in  northeastern 
Minnesota:  Twenty -second  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survej"^  Minn.,  for  1893, 
189i.  pp.  67-78. 

That  part  of  the  region  studied  by  Mr.  Grant,  which  is  included  in 
the  Vermilion  district  as  described  in  this  paper,  lies  in  the  Gunflint  Lake 
area,  north  of  T.  63  N.,  and  between  Rs.  3  and  7  W.  In  Ts.  65  and  66 
N.,  Rs.  4,  5,  and  6  W.,  are  Keewatin  rocks,  including  the  usual  types — 
volcanic  tuff,  greenstone-schists,  greenstone,  and  the  Ogishke  conglomerate. 
The  Saganaga  granite  is  intrusive  in  the  Keewatin,  the  rocks  of  which  it 
metamorphoses.     The  author  feels  himself  justified  in  stating: 

(1)  That  the  rock.s  called  Vermilion  in  the  region  of  the  writer's  field  work  are 
not  necessarily  lower  in  the  geological  scale  than  the  Keewatin,  but  that  thej"  occur 
at  various  horizons  in  the  Keewatin;  (2)  that  thej  are  only  a  more  crystalline 
condition  of  these  same  Keewatin  rocks;  and  (3)  that  they  probably  owe  their  more 
crj'stalline  natui-e  largely  to  their  close  proximity  to  areas  of  intrusive  granite  [p.  71]. 

Th*e  Animikie  iron-bearing  rocks  of  Akeley  Lake  lie  upon  the 
Keewatin  greenstone  to  the  aorth,  and  on  the  south  are  overlain  b}"  the 
Great  gabbro  mass.  The  belt  has  a  width  of  from  300  to  1,300  feet  and 
a  dip  varying  from  20°  to  almost  vertical,  but  averaging  45°  to  50°. 
Where  widest  it  has  an  average  dip  of  30°,  which  would  make  a  maximum 
thickness  of  650  feet.     The  iron  ore  is  a  nontitaniferous  magnetite. 

The  Animikie  rocks  are  little  disturbed,  except  locally,  having  an 
average  dip  of  8°  or  10°  a  little  east  of  south.  The  Animikie  beds 
are  interleaved  with  diabase  sills.     These  give  parallel   east-west  ridges, 


110  THE  VERMILION  IRON-BEARING  DISTRICT. 

wliich  are  gently  sloping  on  the  south  sides,  and  steep  on  tlie  north. 
This  topography  has  led  Lawsou  to  the  conclusion  that  the  apparent 
large  number  of  sills  is  due  to  monoclinal  faulting  of  fewer  layers, 
but  of  this  there  is  no  evidence.  The  Animikie  strata  are  divided  as 
follows:  An  upper  or  graywacke-slate  member,  L,900  feet  thick,  com- 
posed of  slates  and  graywackes,  with  fine-grained  quartzites  and  C|uartz- 
slates;  a  middle  or  black  slate  member,  1,050  feet  thick,  composed  mainly 
of  black  slates,  apparently  carbonaceous,  with  a  fine-grained,  siliceous  and 
flintv  layer  at  the  base  60  feet  thick;  and  a  lower  or  iron-bearing  member, 
composed  largely  of  jaspery,  actinolitic,  siliceous,  and  magnetitic  slates, 
usuallv  thinly  laminated,  and  some  beds  of  cherty  iron  carbonate. 

The  Akelev  Lake  rocks,  first  called  Pewabic  quartzite,  are  similar  to  the 
Gunflint  iron-bearing  rocks,  and  different  from  the  Pewabic  quartzite  and 
conglomerate  found  at  the  base  of  the  Animikie  farther  west  on  the  Mesabi 
range.  From  the  new  data  obtained,  the  Akeley  Lake  iron-bearing  rocks, 
which  rest  directly  upon  the  Keewatin,  are  placed  as  the  iron-bearing 
member  above  the  Pewabic  quartzite. 

Elftjiax.  a.  H.     Preliminary  report  of  field  work  during  1893  in  northeastern 
Minnesota:     Twentv-second  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  of  Minn.,  for 

1893.  1894,  pp.  141-180. 

This  contains  manv  details  concerning  the  structure  and  character  of 
the  rocks  north  and  west  of  Snowbank  Lake.  A  section  is  given  from 
iloose  Lake  tt)  Snowbank  Lake,  showing  relations  of  rocks  in  the 
intervening  area  as  detei'mined  b}'  him. 

Interest  centers  in  the  porphyry  and  granite.  The  porphyry  is  the 
oldest  eruptive.  It  is  found  sending  long  apophyses  across  the  strike  of 
the  Keewatin  rocks,  and  contorting  and  metamorphosing  them.  On  Snow- 
bank Lake  there  are  two  granites,  a  red  hornblende-  and  a  gray  augite- 
granite,  formerly  known  as  red  syenite  and  gray  syenite,  respectively, 
which  are  considered  as  having  been  derived  from  parts  of  the  same 
magma.  The  gray  augite-granite  is  not  found  in  contact  with  the  sedi- 
ments. This  augite-granite,  the  porphyry  referred  to  above,  and  also  the 
sediments  are  cut  by  the  hornblende-granite.  Where  the  sediments  are 
cut  bv  the  granite  thev  are  metamorphosed  to  schists. 

In  foniiectioii  with  the  precedino-  it  iniyht  be  of  interest  to  note  that  the 
hornlilende  and  mica  schists  of  Snowbank  and  White  Iron  lakes  grade  into  argilla- 


EESUME  OF  LITERATUEE.  Ill 

ceous  slates  and  conglomerates.  The  schistose  character  is  most  fully  developed  at 
the  contact  with  the  granite.  All  evidence  tends  to  show  that  the  schists  are  due 
to  the  intrusion  of  the  granite,  and  suggests  that  the  narrow  belts  of  schist  generally 
found  between  the  granite  and  the  Keewatin  rocks,  and  which  have  hitherto  been 
designated  as  a  separate  formation  (Coutchiching  or  Vermilion)  are  only  altered 
portions  of  the  Keewatin,  which  have  been  subjected  to  the  heat  and  action  of  the 
intrusive  granite  [p.  159]. 

The  author  also  adduces  further  evidence  to  prove  that,  in  accord  with 
Dr.  Grant's  statement  to  the  same  effect,  the  so-called  Pewabic  quartzite 
between  Birch  and  Gunflint  lakes  belongs  in  reality  to  the  middle  iron- 
bearing  member  of  the  Animikie.     (Cf  abstract  of  Grant's  report  above.) 

189.5. 

Smyth,  H.  L.,  and  Finlat,  J.  Ralph.  The  geological  structure  of  the  western 
part  of  the  Vermilion  range,  Minnesota:  Trans.  Am.  Inst.  Min.  Engineers,  Vol. 
XXV,  1895,  pp.  595-645. 

Smyth  and  Finlay  describe  the  western  part  of  the  Vermilion  range. 
The  sedimentary  rocks  fall  into  two  divisions.  The  older  is  a  fragmental 
slate  formation,  while  the  younger  is  an  iron-bearing  formation  litholog- 
ically  identical  with  certain  phases  of  the  lower  iron-bearing  formation  of 
the  Marquette  district.  To  all  apjaearances  it  is  devoid  of  clastic  material. 
It  is  believed,  from  analogies  with  other  iron-bearing  districts  of  the  Lake 
Superior  region,  that  the  jaspe)'  of  the  Vermilion  district  is  derived  from  a 
cherty  iron  carbonate  or  from  a  glauconite  greensand,  or  both.  However, 
as  the  jasper  is  a  final  product  of  the  alterations,  it  is  not  possible  to  show 
this. 

Intrusive  igneous  rocks  are  very  abundant,  cutting  or  being  interleaved 
with  the  sedimentary  rocks  in  masses  running  from  the  thickness  of  a  knife 
blade  to  those  100  feet  across.  In  qaantity  the  igneous  rocks  exceed, 
perhaps,  several  times  the  sedimentary  rocks.  The  oldest  igneous  rocks 
are  greenstones.  These  vary  from  massive  to  schistose,  and  in  some 
places  are  what  is  called  conglomei'ate  breccias.  The  acid  rocks  were 
intruded  later  than  the  basic  rocks.  They  were  originally  for  the  most 
part  quartz-porphyries,  but  these  have  been  extensively  changed  to  sericite- 
schists  and  conglomerate  breccias  and  to  rocks  intermediate  between  these 
and  the  original    form.     Within  the  larger  masses  of  the  igneous  rocks, 


112  THE  VERMILION  IRON-BEARING  DISTRICT. 

botli  basic  and  acid,  are  frequently  included  fragments  from  both  the 
slate  and  iron  formations,  from  those  of  small  size  to  masses  more  than 
100  feet  long. 

The  conglomerate  breccias  are  of  dynamic  origin.  The  first  step  in 
the  development  of  the  breccias  was  the  formation  of  two  intersecting  sets 
of  planes  of  fracture,  dividing  the  originally  massive  rocks  into  roughly 
rhomboidal  l^locks.  Their  further  development  depended  on  continued 
movement  between  these  blocks  imder  pressure,  which  resulted  in  enlarging 
the  shearing  zones  at  the  surfaces  of  contact,  and  rounding  the  angles. 
The  slate  and  jasper  inclusions  originall}^  plucked  off  from  the  rocks  which 
the  porphyries  and  greenstones  invaded  shared,  of  course,  the  subsequent 
history  of  their  captors.  The  fact  that  the  jasper  inclusions  are  frequently 
rounded,  while  those  of  slate  are  not,  is  explained  by  the  difference  in  the 
elasticity  of  the  two  rocks.  The  slate  inclusions  readily  yielded  and  finally 
took  a  permanent  set  under  the  deforming  forces,  while  the  harder  and 
more  rigid  jasper,  in  fragments  of  limited  size  and  diverse  orientation, 
behaved  like  the  inclosing  porphyry.  Tlie  boundaries  of  the  inclusions 
were  geuerall}"  the  surfaces  along  which  rupture  took  place,  although,  as 
has  already  been  said,  jasper  in  a  few  instances  is  found  partly  held  in 
porphvry  inclusions. 

As  to  structure,  the  main  slate  area  is  anticlinal ;  both  north  and  south 
of  this  area  the  jasper  succeeds  the  slates.  The  southern  jasjjer  continues  in 
a  complex  syncline,  and  south  of  this  is  found  the  northern  limb  of  another 
anticline  of  slates,  the  southern  limb  not  being  exposed.  Still  farther  south 
is  the  jasper  of  Lee  and  Tower  hills,  which  appears  to  form  the  southern 
and  western  edges  of  a  complex  syncline.  All  of  these  folds  pitch  toward 
the  east. 

The  ore  deposits,  a  number  of  figures  of  which  are  given,  as  well  as 
many  details  concerning  them,  are  found  to  conform  in  occurrence  to  the 
laws  worked  out  by  Van  Hise  in  reference  to  other  districts  of  the  Lake 
Superior  region;  that  is,  (1)  they  occur  for  the  most  part  in  pitching 
troughs  having  impervious  basements,  the  basement  being  usually  one  or 
more  of  the  different  varieties  of  the  eruptive  rocks;  (2)  they  are  secondary 
concentrations  produced  by  downward-percolating  waters,  the  silica  being- 
leached  out  and  the  iron  deposited. 


RESUME  OF  LITERATURE.  113 

1896. 

Elftman,  Arthur  H.  Ore  deposits  of  Minnesota:  Engineers'  Year  Book, 
Univ.  of  Minn.,  1S96,  pp.  115-117. 

This  is  a  brief  statement  of  the  ores  known  to  exist  in  the  State.  Of 
a  number  mentioned,  the  iron-ore  deposits  are  the  only  ones  which  have 
been  developed  lo  any  extent.  These  deposits  occur  in  the  Vermilion  and 
Mesabi  ranges.  In  the  Vermilion  the  -ore  is  a  hematite,  with  low  content 
of  phosphorus  and  sulphur,  rang-ing-  from  soft  to  hard  ore.  The  deposits 
are  in  the  Keewatin  of  the  Lower  Huronian,  and  are  mined  only  at  Tower 
and  Ely.  Eastward  from  Ely,  extending-  tlu'ough  the  eastern  part  of 
Hunters  Island,  are  very  favoi'able  indications  of  ore  deposits. 

1897. 

Eby,  J.  H.,  and  Berket,  Chas.  P.  Co^jper  minerals  in  hematite  ore:  Engi- 
neers' Year  Book,  Univ.  of  Minn.,  1897,  pp.  108-117.  (Rejarinted  from  Proc.  Lake 
Superior  Min.  Inst.) 

Mr.  Eby  describes  the  occurrence  of  a  number  of  copper  minerals  in 
the  hematite  ore  of  the  Montana  mine,  of  Soudan,  Minn.  The  minerals 
associated  with  the  native  copper  found  in  the  hematite  are  cuprite, 
malachite,  azurite,  and  chalcopyrite.  The  native  copper  seems  to  have 
been  the  source  of  the  copper  minerals,  as  in  one  case  an  octahedron  was 
found  which  consisted  of  metallic  copper  at  center,  surrounded  by  layers 
of  cuprite  (CuO),  and  this  surrounded  by  copper  carbonate. 

Mr.  Berkey  describes  the  minerals,  excepting  the  chalcopyrite,  which 
was  not  found  in  the  specimens  he  had  for  study. 

WiNCHELL,  N.  H.  Some  new  features  in  the  geology  of  northeastern  Minnesota: 
Am.  Geologist,  Vol.  XX,  1897,  pp.  41-51. 

Winchell  presents  some  additional  points  on  the  geology  of  north- 
eastern Minnesota. 

The  Laurentian  includes,  in  Minnesota,  an  acid  crystalline  schist  of 
sedimentary  origin  and  a  massive  igneous  rock,  although  the  igneous  rock 
is  younger  than  the  crystalline  schist  portion  and  should  have  a  different 
desig'nation.  The  conclusions  reached  are  that  (1)  the  sedimentary  Lau- 
rentian is  a  crystalline  condition  of  sedimentary  strata,  which  are  con- 
formably a  portion  of  the  sedimentary  schists;  (2)  the  igneous  Laurentian 

MON  XLV — 03 8 


114  THE  VEKMILION  IRON-BEARING  DISTRICT. 

is  the  result  of  a  more  intense  metamorphisra,  carried  even  to  fusion  of 
some  sti-ata.  These  conclusions  result  particularly  from  the  study  of  a 
section  from  Tower  northward  through  Vermilion  Lake,  and  of  an  area  on 
the  west  side  of  Outlet  Bay,  in  the  corners  of  sees.  13,  14,  21,  and  32,  T. 
63  N.,  R.  1 7  W.,  and  along  the  shore  'for  one-half  mile  westward. 

It  is  evident  that  the  Stuntz  conglomerate  on  the  south  shore  of  Ver- 
milion Lake  is  a  true  water-deposited  conglomerate  of  the  same  fonnation 
as  the  slates  and  graywackes  of  the  district,  the  conglomerate  grading 
into  the  quartzite  and  graywacke,  and  this  into  argillaceous  slate.  Further- 
more, as  supposed  by  Van  Hise,  the  conglomerate  lies  uncoufonnably  on 
the  iron-bearing'  formation,  and  contains  very  numerous  fragments  of 
jaspilite.  The  jjosition  of  this  unconformity,  whether  at  the  base  of  the 
Taconic  or  lower,  is  not  ascertained. 

1898. 

Grant,  U.  S.  Sketch  of  the  geology  of  the  eastern  end  of  the  Mesabi  iron 
i-ange  in  Minnesota:  Engineers'  Year  Book,  Univ.  of  Minn.,  1898,  pp.  49-62; 
with  sketch  map. 

Grant  sketches  the  geology  of  the  eastern  end  of  the  jMesabi  iron  range 
in  Minnesota,  including  T.  64  N.,  Rs.  3  and  4  W.,  and  parts  of  Rs.  2  and 
5  W.,  with  some  adjacent  portions  of  Ontario.  The  rocks  can  be  separated 
into  three  divisions.  The  chief  one  of  these  is  the  Animikie  series,  contain- 
ing the  iron-bearing  rocks  of  the  Mesabi  range.  Older  than  the  Animikie 
is  a  senes  of  granites,  greenstone  both  massive  and  schistose,  conglomerates, 
slates,  and  other  clastic  rocks,  called  the  pre- Animikie.  Younger  than  the 
Animikie  are  some  diabase  sills  and  the  Great  gabbro  mass  of  noi'theastern 
Minnesota. 

Of  the  pre-Animikie  rocks,  the  greenstones  and  clastic  rocks  have  been 
called  Keewatin.  As  the  greenstones  are  usually  associated  with  the 
Mesabi  iron-bearing  rocks,  these  alone  of  the  Keewatin  rocks  are  described. 
They  lie  to  the  north  of  the  iron-bearing  rocks  in  T.  65  N  ,  R.  5  W.,  and 
extend  eastward  to  the  center  of  T.  65  N.,  R.  4  W.,  where  they  disappear 
under  the  Animikie  strata.  In  general,  the  greenstones  are  at  present 
diorites;  originally  some  were  certainly  diabases,  others  were  of  the  nature 
of  andesites,  and  a  large  part  were  diorites  or  possibly  gabbros.  At  places, 
especially  along  the  east  side  of  sec.  27,  T.  65  N.,  R.  5  AY.,  the  greenstones 


RESUME  OF  LITERATURE.  115 

contain  angular  and  subangular  fragments  of  rock  almost  like  themselves, 
and  some  may  be  regarded  as  composed  of  fragmental  volcanic  rocks. 
Associated  with  the  greenstones,  especially  in  sees.  22,  23,  and  24,  T.  65 
N.,  R.  5  W.,  are  small  masses  of  more  acid  rocks,  quartz-porphyries,  and 
quartzless  porphyries,  which  are  probably  younger  than  the  greenstones. 

The  pre-Animikie  granite  has  its  typical  development  on  the  shores  of 
Saganaga  Lake.  In  a  number  of  places  it  may  be  seen  in  intrusive 
relations  with  the  greenstone.  A  quarter  of  a  mile  south  of  the  northeast 
corner  of  sec.  23,  T.  65  N.,  R.  5  W.,  many  granite  dikes  cutting  the  green- 
stone are  seen,  and  on  the  south  shore  of  West  Sea  Gull  Lake  granite  dikes 
of  the  same  nature  as  the  immediately  adjacent  main  mass  of  granite  of 
Saganaga  Lake  are  seen  cutting  the  greenstone.  Both  granite  and  green- 
stone are  cut  by  another  series  of  finer-grained,  more  acid  granite  dikes. 

The  Animikie  rocks  rest  uncomformabl}^  upon  the  pre-Animikie  rocks, 
and  are  usually  exposed  on  the  south  slope  of  the  Giants  range,  which  is 
composed  essentially  of  granite.  The  strike  is  approximately  east-northeast, 
and, the  dip  in  general  about  10°  SE.  The  thickness  varies  from  nothing 
to  4,000  feet.  The  Animikie  is  separable  into  four  conformable  divisions — 
(1)  the  lower  or  qnartzite  member,  called  the  Pewabic  quartzite;  (2)  the 
iron-bearing  or  taconite  member;  (3)  the  black-slate  member;  (4)  the 
graywacke-slate  member. 

(1)  The  quartzite  member  is  well  developed  in  Itasca  County,  but 
disappears  before  reaching  the  eastern  side  of  St.  Louis  County. 

(2)  The  rocks  of  the  iron-bearing  member  are  similar  to  those  in  St. 
Louis  County  on  the  western  end  of  the  range,  described  by  Spun-."  They 
differ,  however,  in  two  features.  They  are  more  completely  crystalline, 
and  the  iron  is  magnetite  instead  of  hematite.  The  rocks  consist  chiefly 
of  jaspers,  amphibole-  (griinerite)  schists,  greenish  siliceous  slates,  cherts, 
cherty  carbonates,  and  magnetite  slates.  It  is  believed  that  these  rocks 
were  originally  glauconitic  greensands;  that  the  ore  has  been  derived  from 
the  iron  in  the  glauconite,  and  that  the  ore  bodies  result  from  concentration 
and  replacement.  In  this  part  of  the  Mesabi  range  no  ore  bodies  have 
yet  been  found  which  are  at  the  same  time  both  rich  enough  and  large 
enough  for  profitable  mining,   although  vast  quantities  of  magnetite  ore 

«Geol.  and  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  10,  1894. 


116  THE  VERMILION  IRON-BEARING  DISTRICT. 

occur  at  or  near  the  surface.  The  dip  of  this  formation  varies  from  an 
average  of  45°  to  50°  on  the  west  to  less  than  16°  on  the  east,  and  the 
thickness  varies  from  650  feet  or  less  on  the  west  to  900  feet  on  the  east. 

(3)  The  black  slate  is  essentially  a  fine-grained,  black,  more  or  less 
siliceous,  apparentl}-  carbonaceous  slate. 

(4)  The  graywacke-slate  member  is  composed  of  black  to  gray  slates 
and  fine  graywackes,  with  some  flinty  slates;  the  upper  part  shows  coarser 
detrital  material,  and  the  highest  beds  seen  are  fine-grained  quartzites  and 
quartz-slates.  This  member  is  well  exposed  on  the  south  shore  of  Loon 
Lake. 

Associated  with  all  of  the  strata  of  the  Animikie  are  diabase  sills,  and 
bounding  the  Animikie  rocks  on  the  south  is  the  Grreat  gabbro  mass.  These 
are  igneous  rocks  of  later  date  than  the  Animikie.  Near  the  contact  with 
the  gabbro  the  Animikie  rocks  show  marked  metamorphism  and  usually 
complete  recrystallization.  The  gabbro  varies  from  a  nearly  pure  plagio- 
clase  rock  to  titaniferous  magnetite. 

The  pre-Auimikie  rocks  here  described,  according  to  the  nomenclature 
used  by  the  United  States  Geological  Survey,  belong  to  the  Lower  Huronian 
series  of  the  Algonkian  system,  and  probably  also  in  part  to  the  older 
Archean  or  Basement  complex;  the  Animikie  is  regarded  as  the  equivalent 
of  the  Upper  Huronian  series  of  the  Algonkian,  and  the  gabbro  as  a  part 
of  the  Keweenawan  series  of  the  Algonkian. 

1S99. 

Sardeson,  F.  W.  Report  of  secretary  of  the  Geological  Club  of  the 
University  of  Minnesota:  Science,  Vol.  IX,  pp.  -412-413. 

Prof  C.  W.  Hall  discusses  "The  extent  and  distribution  of  the  Archean 
in  Minnesota."     The  following  quotations  are  from  the  secretary's  report: 

Accepting  the  Archean  as  that  original  "crust"  or  solidified  portion  of  the 
earth,  ...  he  defined  it  as  an  era  of  igneous  origins,  whose  rocks  represent  the 
original  crystallization  of  earth  matter  added  to  from  below  by  successive  solidifica- 
tion and  many  subsequent  intrusions.  By  this  definition  all  overlying  elastics  or 
irruptions  into  or  through  the  elastics  are  excluded  from  the  Archean.  .  .  . 

Between  Rainy  Lake  and  Lake  Superior  there  are  several  belts  of  schists,  with 
alternating  granites  and  other  rocks  having  a  general  northeast-and-southwest  trend. 
Concerning  one  of  these,  Irving  noted  in  ISSfi  "that  we  have  among  the  rocks  .  .  . 
two  types,  in  one  of  which  the  crystalline  structure  is  complete  and  in  which  there  is 


RESUME  OF  LITERATURE.  117 

little  or  none  of  an  original  fragraental  structure,  while  in  the  other  the  fragmental 
texture  is  still  distinct  and  the  alteration  has  progressed  to  a  smaller  degree."  He 
then  adds  "that  the  supposed  older  one  of  the  two  groups  of  schists  in  the  Vermilion 
Lake  belt  is  intricatelj'  penetrated  by  the  granites  of  the  great  areas  north  and  south 
of  the  belt.""     Hence  areas  of  Archean  lie  north  and  soutli  of  these  older  schists. 

It  is  clear  that  Professor  Hall  believes  in  the  presence  of  Arohean  rocks 
in  the  Vermilioji  district,  although  the  careful  reader  will  see  that  Professor 
Hall's  conclusion  as  to  their  occurrence  there  does  not  follow  from  his  defini- 
tion of  Archean  as  reported  here  and  from  the  quotation  from  Professor 
Irving's  report. 

WiNCHELL,  N.  H.,  Grant,  U.  S.,  and  Elftman,  A.  H.  Twenty-fourth  Annual 
Report  Geol.  and  Nat.  Hist.  Survey  of  Minnesota  for  years  1895-1S9S,  1899. 

In  this  annual  report,  which  it  is  stated  is  the  final  one,  there  are  pub- 
lished the  lists  of  rock  specimens,  with  annotations,  collected  by  N.  H. 
Winchell  in  1896,  1897,  1898;  a  record  of  the  field  work  of  U.  S.  Grant, 
1892-1898,  and  a  list  of  specimens  collected  by  him  in  1898;  also  a  list  of 
specimens  collected  by  A.  H.  Elftman  in  1895,  1896,  1897. 

In  these  we  find  statements  concerning  the  Vermilion  district,  but  the 
material  is  not  digested,  and  no  general  conclusions  are  stated.  Conse- 
quently it  is  impracticable  to  review  the  report  and  show  its  actual  contents. 

Winchell,  N.  H.,  et  al.  The  geology  of  Minnesota,  by  N.  H.  AVinchell,  U.  S. 
Grant,  James  E.  Todd,  Warren  Upham,  and  H.  V.  Winchell:  Final  Report  Geol. 
and  Nat.  Hist.  Survey  of  Minn.,  Vol  IV,  1899,  pp.  630.     With  31  geologic  plates. 

Structural  geology  of  Minnesota,  by  N.  H.  Winchell:  Final  Report  Geol.  and 
Nat.  Hist.  Survey  of  Minn.,  Vol.  V,  1900,  pp.  1-80,  972-1000. 

The  first  of  these  volumes  contains  an  account  of  detailed  field  work 
in  northeastern  Minnesota,  with  incidental  discussion  of  general  problems. 
The  area  is  treated  by  counties  and  smaller  arbitrary  geographic 
divisions,  in  the  description  of  which  several  men  have  taken  part.  This 
manner  of  treatment  leads  to  repetition  in  the  discussion  of  the  general 
geologic  features,  and  in  many  cases  it  is  extremely  difficult  to  correlate 
the  facts  recorded  in  the  different  sections. 

Volume  V  contains  an  account  of  the  general  structural  geology  of' 
the  State,  by  Professor  Winchell,  based  on  the  detailed  work  described  in 


a  Seventh  Ann.  Rept.  U.  S.  Geol.  Survey,  p.  437. 


118  THE  VERMILION  IRON-BEAKING  DISTRICT. 

\'(il.  IV.  This  general  discussiou  of  Vol.  V  is  reviewed,  with  such 
reference  to  the  facts  recorded  in  Vol.  IV  as  is  necessary  to  make  the 
summary  intelligible. 

Dr.  Grant's  views,  as  indicated  in  the  detailed  descriptions  of  special 
areas,  in  some  cases  differ  somewhat  widely  from  those  of  Professor 
Winchell. 

Winchell  discusses  the  general  structural  geology  of  northeastern 
Minnesota.  The  ancient  rocks  of  this  area  he  places  in  two  main  systems, 
the  Archean  and  the  Taconic.  Tlie  former  is  further  subdi-\dded  into  the 
Upper  and  Lower  K  eewatiu,  separated  from  each  other  b}'  an  unconformity. 
The  Pewabic  quartzite  also  is  placed  with  the  Keewatin,  but  is  not 
assigned  to  either  of  the  main  divisions.  Overlying  the  Archean  with 
strong  unconformity  is  the  Taconic,  represented  by  Animikie  and  Kewee- 
nawan  rocks,  these  divisions  being  supposed  to  represent,  I'espectively, 
the  Lower  and  Middle  Cambi-ian  of  other  parts  of  the  country.  The 
Coutchiching  and  Laurentian  rocks  before  mapped  as  separate  formations 
are  now  included  within  the  Keewatin. 

The  Lower  Keewatin  comprises  greenstone,  with  associated  surface 
volcanics  which  are  both  subaerial  and  subaqueous,  argillitic  slates,  siliceous 
schists,  quartzites,  arkoses,  "greenwackes,"  iron  ores,  and  marble. 

The  greenstone,  designated  the  Kawishiwin,  is  the  oldest  known  rock 
in  the  State,  and  is  supposed  to  represent  a  portion  of  the  original  crust  of 
the  earth.  With  its  associated  volcanic  rocks  it  occurs  in  two  main  belts. 
The  southern  belt  begins  in  the  vicinity  of  Gunflint  Lake  and  extends 
indefinitely  westward  b}^  way  of  Gobbemichigamma  Lake,  the  Kawishiwi 
River,  White  Iron  Lake,  and  Tower.  The  northern  belt  of  greenstone 
enters  the  State  from  Hunters  Island,  appearing  conspicuously  at  the  south 
side  of  Basswood  Lake.  At  Pipestone  Rapids  and  Fall  Lake  it  widens 
southward  and  appai'ently  unites  at  the  surface  with  the  southern  belt,  the 
overlying  Upper  Keewatin  being  absent  for  a  distance  of  a  few  miles.  But 
farther  west  it  is  again  divided  by  the  Stuntz  conglomerate,  the  northern 
arm  running  to  the  north  of  Vermilion  Lake,  west  of  which  its  extension 
is  unknown,  and  the  southern  one  running  south  of  the  lake. 

The  fragmental  stratified  rocks  of  the  Lower  Keewatin  are  most  impor- 
tant toward  the  western  part  of  the  area  of  exposure  of  crystalline  rocks. 
They  occupy  a  wide  area  south,  west,  and  north  of  Tower.     The  iron  ores 


RESUMfi  OF  LITERATURE.  119 

of  Tower  and  Ely  ou  the  Yermiliou  iron  range  occi;r  in  tlie  upper  part  of 
the  Lower  Keewatin.  It  is  probable  that  the  immediately  inclosing'  rock 
is  a  sedimentary  one,  although  composed  of  the  elements  of  a  basic  erup- 
tive. The  sediments  extend  south  to  the  Giants  range  of  granite,  where 
they  are  metamorphosed  to  mica- schists  by  the  granite.  Toward  the  west 
they  extend  as  far  as  the  Mississippi  River  and  its  northern  tributaries  and 
across  the  Bowstring,  although  the  drift  prevents  the  delimitation  of  the 
belt.  To  the  northwest  they  extend  toward  Rainy  Lake,  in  this  direction 
being  converted  into  mica-schists  and  gneisses  by  the  intrusion  of  granite; 
in  unmodified  form  they  are  found  at  one  point  only  on  Rainy  Lake. 
These  fragmental  rocks  of  the  Lower  Keewatin  doubtless  also  underlie 
most  of  the  central  and  southwestern  part  of  the  State  as  far  as  the  Minne- 
sota River.  Here  thev  dip  l^eueath  the  later  formations  in  the  southwestern 
portion  of  the  State,  and  probably  occupy  a  wide  patch  in  South  Dakota. 
South  of  the  Giants  range  they  occur  also,  but  as  they  are  covered  by  the 
gabbro  and  Animikie  toward  the  east  and  the  di'ift  deposits  of  the  St.  Louis 
Valley  toward  the  west  their  geographic  boundaries  are  mostly  unknown. 
They  appear  in  the  central  and  western  portions  of  Carlton  County,  where 
their  line  of  separation  from  the  Upper  Keewatin  is  quite  obscure,  and  in 
the  central  and  western  portions  of  Morrison  County.  The  Lower  Keewatin 
marble  is  seen  at  Ogishke  Muncie  Lake  and  at  Pike  Rapids  on  the 
Mississippi. 

The  Lower  Keewatin  was  terminated  by  a  period  of  extensive  folding 
and  intrusions  of  granite  and  basic  rocks. 

The  Pewabic  quartzite  belongs  with  the  Keewatin,  but  whether  to  the 
Lower  or  Upper  Keewatin  is  not  known.  This  formation  includes  altered 
quartzites  and  iron  ores  between  the  granite  and  gabbro  in  the  immediate 
vicinity  of  Birch  Lake  and  small  patches  of  similar  rocks  in  sec.  30,  T.  62 
N.,  R.  10  W.;  on  the  soiith  shore  of  Disappointment  Lake;  on  the  north 
shore  of  Fraser  Lake;  on  the  south  shore  of  Gobbemichigamma;  at  Akeley 
Lake,  forming  the  so-called  Akeley  Lake  series  extending-  from  the  west  side 
of  sec.  34,  T.  65  N.,  R.  5  W.,  to  the  eastern  part  of  sec.  27,  T.  65  N.,  R.  4  W. 

The  Upper  Keewatin  occurs  in  troughs  in  the  Lower  Keewatin,  and 
particularly  in  one  main  trough  the  axis  of  which  is  traceable  from  Vermil- 
ion Lake  to  Saganaga  Lake.  The  northern  arm  of  this  syncline,  consist- 
ing of  granites,  gneisses,  associated  mica-schists,  and  in  some  places  earlier 


120  THE  VERMILION  IRON-BEARING  DISTRICT. 

greenstones,  extends  from  the  northern  part  of  Vermilion  Lake  through 
Basswood  Lake  to  the  northern  side  of  Hunters  Island.  The  southern 
arm,  consisting  of  Lower  Keewatin  green  schists  and  other  schists,  pene- 
trated by  the  granite  of  the  Giants  range,  extends  from  Pokegama  Falls  on 
the  southwest  toward  the  northeast,  until  cut  out  by  the  encroachment  of 
the  gabbro  from  the  south.  The  Upper  Keewatin  consists  very  largely  of 
conglomerates,  but  also  includes  graywackes,  argillites,  quartzites,  and 
jaspilites,  in  general  coarser  than  those  of  the  Lower  Keewatin.  Volcanic 
rocks  are  less  important  than  in  the  Lower  Keewatin,  although  still 
present.  There  is  no  general  order  of  succession  in  the  Upper  Keewatin 
excepting  that  it  can  be  said  that  it  is  in  general  conglomeratic  at  the  bottom. 

After  Upper  Keewatin  time  both  the  Lower  and  Upper  Keewatin  were 
subjected  to  another  folding,  the  axis  of  which  had  a  general  parallelism 
with  the  earlier  folding,  with  the  result  that  the  Upper  Keewatin  lies  in 
narrow  S5niclines  in  the  Lower  Keewatin  and  in  places  is  nearly  or  quite 
vertical. 

Associated  with  the  Keewatin  rocks  are  granites  of  at  least  two  periods 
of  intrusion,  one  later  than  the  Lower  Keewatin  and  one  later  than  the 
Upper  Keewatin.  The  later  granite  is  believed  to  be  represented  by  the 
hio-her  parts  of  the  Giants  range  and  the  Snowbank  Lake  granite.  The 
earlier  granite  is  represented  by  the  granites  at  Kekequabic  (Cacaquabic) 
Lake,  Saganaga  Lake,  Basswood  Lake,  Burntside  Lake,  Vei-milion  Lake, 
Lac  la  Croix,  and  Kabetogama  Lake.  The  origin  of  the  granite  is  dis- 
cussed and  the  same  conclusions  reached  as  in  a  previous  article." 

The  laconic. — This  is  unconformably  above  the  Keewatin  rocks.  It 
comprises  the  Animikie  and  Keweenawan  divisions. 

The  Animikie  rocks  enter  the  State  at  Pigeon  Point,  run  westward  along 
the  international  boundary  to  the  eastern  part  of  sees.  22  and  27,  T.  65  N., 
R.  4  W.  They  reappear  again  south  westward  from  Birch  Lake  on  the  north- 
west side  of  the  gabbro  mass,  and  thence  continue  along  the  south  side  of 
the  Giants  range,  constituting  the  IMesabi  iron  series,  to  Pokegama  Falls. 
The  hio-her  parts  of  the  Animikie  are  best  developed  toward  the  east  while 
the  lower  parts  are  best  developed  toward  the  west. 

"The  origin  of  the  Archean  igneous  rocks,  by  N.  H.  Winchell:  Proc.  Am.  Assoc.  Adv.  Sci.,  Vol. 
XLVII,  1898,  pp.  .303,  ,304  (Abstract).  Also  Am.  Geologist,  Vol.  XXII,  1898,  pp.  299-310.  Summar- 
izeil  Jour.  Geol.,  Vol.  VII,  1899,  p.  194. 


RESUME  OF  LITERATURE.  121 

The  Animikie  rocks  comprise  the  Pokegama  quartzite,  Mesabi  iron- 
bearing  formation,  some  hmestone  and  slate,  all  stricth'  conformable  with 
one  another.  The  thickness  is  several  hundred  feet,  sometimes  reaching 
nearly  1,000  feet.     The  dip  of  the  series  is  miiformly  to  the  south,  8°  to  12°. 

The  iron-bearing  formation  and  the  Pokegama  quartzite  constitute  the 
base  of  the  formation.  The  quartzite  in  places  is  beneath  the  iron  forma- 
tion; in  other  places  it  is  in  the  some  horizon,  and  in  still  others  is  above 
the  iron  formation.  Commonly  the  base  of  the  Animikie  is  marked  by  a 
conglomerate,  containing  debris  from  the  underlying  Keewatin  rocks. 
This  is  a  narrow  horizon  which  soon  graduates  upward  into  a  quartzite, 
known  as  the  Pokegama  quartzite,  from  its  typical  development  near  Poke- 
gama Falls  on  the  Mississippi  River.  The  thickness  of  the  quartzite  is  not 
known  to  exceed  50  feet,  and  is  sometimes  less  than  25  feet. 

Above  the  quartzite,  or  in  alternating  beds  with  it,  or  below  it,  appears 
the  iron-bearing  or  taconite  member  of  the  Animikie,  which  contains  the 
iron-ore  deposits  of  the  Mesabi  iron  range.  The  ore  is  usually  hematite  in 
the  western  part  of  the  range  and  magnetite  in  the  eastern  part.  It  was 
previously  supposed  to  have  been  derived  from  the  alteration  of  a  greenish 
glauconitic  sand  rock;  but  later  work  has  seemed  to  show  that  the  green- 
sand  is  a  volcanic  sand,  and  that  the  so-called  taconitic  rock  itself  has 
resulted  from  igneous  forces.  This  is  accounted  for  by  supposing  a  chain  of 
active  volcanoes  to  have  existed  where  the  Mesabi  iron  range  is  now  found. 
These  volcanoes  yielded  flows  and  ejectamenta  to  the  adjacent  waters  which 
have  been  modified  into  the  various  phases  of  the  iron  formation  now  seen. 
This  volcanic  epoch  may  have  a  deep-seated  connection  with  the  Cabotian 
or  lower  division  of  the  Keweenawan  (described  later). 

Above  the  iron-bearing  member  is  an  impure  dark  colored  limestone  a 
few  feet  in  thickness,  not  exceeding  20.  It  extends  apparently  the  whole 
length  of  the  Mesabi  range,  but  has  been  identified  in  two  places  only,  sec.  7, 
T.  58  N.,  R  17  VV.,'and  doubtfully  on  the  shores  of  Gunflint  Lake.  This 
limestone  may  be  regarded  as  the  basal  horizon  of  the  next  overlying  rock. 

The  black  slate  is  probably  several  thousand  feet  in  thickness  and 
constitutes  the  bulk  of  the  Animikie.  In  the  neighborhood  of  Gunflint 
Lake  it  has  been  divided  by  Dr.  Grant  into  a  lower  black-slate  division 
and  an  upper  graywacke-slate  division,  both  of  which  members  are  intruded 
bv  diabase  sills 


122  THE  VERMILION  IRON-BEARING  DISTRICT. 

In  the  Indian  reservation  at  Grand  Portage  and  at  various  places  along 
the  Grand  Portage  trail  is  a  graywacke,  which  is  supposed  to  overlie  the 
black-slate  member,  but  its  extent  and  stratigraphic  position  have  not  been 
satisfactorily  established. 

The  top  of  the  Animikie  has  not  been  identified.  The  first  recogniz- 
able datum  jjlane  after  the  close  of  the  Animikie  is  the  Puckwunge 
conglomerate,  supposed  to  be  the  fragmental  base  of  the  Keweenawan. 

At  one  or  two  places  southwestward  from  Birch  Lake,  and  at  Little 
Falls  on  the  Mississippi  River,  and  in  Morrison  County,  the  Animikie  has 
been  converted  into  a  mica-schist. 

The  age  of  the  Animikie  is  believed  to  be  Lower  Cambrian  for  the 
following  reasons:  It  graduates  upward  into  Upper  Cambrian  rocks  as  seen 
on  the  south  side  of  Lake  Superior.  The  derivation  of  the  iron  ores  from 
a  glauconitic  greensand  indicates  that  larg-e  quantities  of  foraminiferal 
orsranisms  once  lived  in  the  Animikie  ocean,  and  Matthew  has  shown  the 
existence  of  foraminiferal  organisms  associated  with  the  iron  ore  in  the 
St.  Johns  group  of  New  Brunswick.  Further,  the  Animikie  has  a  uniformly 
low  dip,  while  the  lower  strata  are  all  highly  tilted.  There  must  therefore 
have  been  a  great  lapse  of  time  between  the  deposition  of  the  two  series. 

The  Keiveenaivan. — The  Puckwunge  conglomerate  is  taken  to  be  the 
fragmental  base  of  the  Keweenawan,  although  certain  igneous  rocks  which 
antedate  it,  and  which,  perhaps,  are  contemporaneous  with  the  upper  por- 
tions of  the  Animikie,  are  also  called  Keweenawan.  The  conglomerate  is 
found  at  Grand  Portage  Island,  at  Isle  Royale,  on  the  Baptism  River,  at 
Little  Marais,  on  Manitou  River,  at  the  deep  well  at  Short  Line  Park,  near 
Duluth,  and  at  New  Ulm. 

Above  this  conMomerate  are  cong-lomerates  and  sandstones  of  Ke- 
weenawau  age  which  are  stratified  with  lavas  of  diabasic  nature.  Still 
higher  up  the  eruptive  rocks  become  less  in  quantity  and  the  fragmental 
rock  is  a  sandstone,  known  as  the  Hinckley  sandstone,  quarried  in  tlie 
gorge  of  the  Kettle  River  in  Pine  County.  This  in  turn  grades  up 
into  typical  Upjjer  Cambrian  sandstones  of  the  St.  Croix  Valley.  The 
term  Potsdam  is  restricted  to  the  Puckwunge  conglomei-ate  and  the 
hardened  quartzites  immediately  overlying  it,  represented  by  the  Sioux 
quartzite,  the  Baraboo  and  Barron  county  quartzites  of  Wisconsin,  the 
quartzite  at    Grand    Portage   Island,  and  west  of  Grand  Portage  village, 


RESUME  OF  LITERATURE.  123 

the  New  Ulm  quartzite  iu  Cottonwood  County,  and  the  quartzite  in  Pipes- 
tone County. 

The  igneous  rocks  of  the  Keweenawau  vary  in  age  from  the  late 
Animikie  time  to  the  top  of  the  Keweeuawan  series.  They  are  divided 
into  two  groups,  the  Cabotian  or  Lower  Keweenawau,  and  the  Mfur't  a  or 
Upper  Keweenawau. 

The  Cabotian  division  inchides  gabbro  and  contemporaneous  red  rock 
and  their  surface  lavas,  and  all  other  dikes  and  sills  which  are  associated 
with,  but  younger  than,  the  Animikie  clastic  rocks,  and  which  are  older 
than  the  Puckwunge  conglomerate.  The  lower  member  of  the  Cabotian  is 
the  gabbro,  which  covers  an  enormous  area.  It  extends  on  the  east  to  East 
Greenwood  Lake,  in  T.  64  N.,  R.  2  W.  On  the  north  it  is  bounded  by  the 
Animikie  strata  of  the  Mesabi  iron  range.  Its  westernmost  exposure  is  in 
the  vicinity  of  Short  Line  Park,  Duluth.  The  southern  limit  is  irregular, 
swinging  from  East  Greenwood  Lake  in  a  zigzag  manner  through  T.  63  N., 
R.  1  W.;  T.  62  N.,  R.  2  W.;  T.  62  N ,  R.  4  W.;  T.  60  N.,  R.  6  W.;  T.  60  N., 
R.  7  W.;  T.  58  N.,  R.  10  W.;  and  T.  55  N.,  R.  11  W.,  to  Duluth. 

Along  the  northern  and  northwestern  side  of  the  Great  gabbro  mass, 
the  gabbro  is  plainl}-  intrusive  on  the  older  formations,  Animikie  and 
Keewatin. 

From  the  northern  border  of  the  gabbro  many  sills  offshoot  and  pene- 
trate the  Animikie  strata  parallel  to  the  bedding.  These  are  known  as  the 
Logan  sills. 

Near  its  contact  with  the  underlying  rocks,  both  the  Animikie  and 
Keewatin  series,  there  are  various  altered  rocks  which  can  be  connected  in 
places  with  the  gabbro  and  in  places  with  the  underlj-ing  rocks.  To  these 
altered  rocks  the  term  muscovadyte  has  been  applied.  It  includes  the 
various  so-called  peripheral  phases  of  the  gabbro. 

On  the  southern  and  eastern  border  the  gabbro  is  penetrated  by  and 
penetrates  in  a  confused  manner  the  red  rock,  with  which  it  alternates  both 
structurally  and  areally.  It  is  believed  to  ha^^e  resulted  from  the  meta- 
morphism  by  the  gabbro  of  the  Animikie,  and  perhaps  earlier  fragmentals. 

As  the  granites  of  the  Archean  are  believed  to  have  resulted  from  the 
softening  of  acid  fragmentals,  so  the  gabbro  may  probably  have  been  the 
result  of  the  metamorphism  or  refusion  of  the  Keewatin  greenstones. 

The  anorthosite  masses  of  the  Beaver  Bay  diabase,  supposed  by  Lawson 


124  THE  VERMILION  IKON-BEARING  DISTRICT. 

to  be  Ai-ehean  and  to  uuclerlie  unconformably  the  Beaver  Bay  diabase,  are 
believed  to  represent  segregation  phases  in  the  main  gabbro  flow,  and  to 
be  the  same  as  anorthosite  masses  in  the  gabbro  proper  to  the  west. 

The  Beaver  Bay  diabase  is  beheved  to  represent  the  upper  portion  of 
the  Great  gabbro  flow,  and  to  be  due  to  the  first  and  greatest  movement  of 
the  gabbro  toward  Lake  Superior.  The  Logan  sills  belong  to  this  part 
of  the  gabbro  flow. 

The  Manitou'  division  of  the  Keweenawan  includes  the  surface  flows, 
sills,  and  dikes  which  accompanied  and  followed  the  Puckwunge  conglom- 
erate. These  eruptives,  with  the  elastics  associated  with  them,  do  not  have 
a  thickness  in  Minnesota  of  more  than  1,000  feet.  These  lava  sheets  extend 
along  the  shore. of  Lake  Superior  from  near  Baptism  River  to  near  Grand 
Marais,  except  where  replaced  at  intervals  by  the  Beaver  Bay  diabase  or 
some  of  the  intersheeted  fragmentals.  They  occur  also  in  the  neighborhood 
of  Grand  Portage  Bay,  but  their  extent  here  is  not  definitely  known. 

General. — The  most  important  petrologic  conclusions  reached  from  the 
examination  of  the  Minnesota  crystalline  rocks  are  three  in  number: 

1.  All  the  granites  of  the  Archean  can  be  explained  on  the  assumption 
that  they  are  intrusives  representing  the  metamorphosed  conditions  of 
clastic  rocks  adjacent  to  the  observed  intrusions,  rendered  plastic  by  the 
force  of  dynamic  metamorphism  accompanied  by  moisture. 

2.  The  Keweenawan  gabbro  and  its  derivatives  are  derived  from  the 
metamorphism  and  refusion  of  the  Archean  greenstones  and  their  attendants. 

Comment. — The  two  main  petrologic  conchasions  announced  by  Pro- 
fessor Winchell  as  the  most  important  results  of  his  final  petrologic  work, 
summarized  in  the  closing  general  paragraph,  would  be  dissented  from  by 
most  of  the  other  geologists  who  have  worked  in  this  area." 

The  Cambrian  age  of  the  Animikie  strata  has  long  been  maintained  by 
Professor  Winchell,  and  above  are  summarized  his  arguments  in  support  of 
this  position.  The  first  argument,  that  the  Animikie  grades  into  the  Upper 
Cambrian  rocks,  is  not  in  accord  with  the  observations  of  most  of  the 
geologists  above  referred  to.  The  second  argument,  based  on  the  similarity 
of  the  unaltered  greensand  in  the  Mesabi  district  to  that  in  the  Cambrian 
of  the  eastern  United  States,  loses  weight  when  we  consider  the  fact  that 
the  similarity  is  not  great,    the  differences  being   many  and    significant; 

"Some  of  these  geologists  are:  R.  D.  Irving,  C.  R.  Van  Hise,  J.  Morgan  Clements,  W.  S.  Bayley, 
U.  S.  Grant,  J.  E.  Spurr,  A.  H.  Elftman,  C.  K.  Leith. 


RESUME  OF  LITERATURE.  125 

and  if  the  similarity  were  complete,  the  correlation  would  involve  laying 
too  much  stress  on  lithologic  similai-it}^  of  widely  separated  formations. 
Professor  Winchell's  latest  conclusion,  that  the  Mesabi  greensand  is 
volcanic  and  not  organic,  while  entirely  dissented  from  by  others  who  have 
studied  this  rock,  in  itself  spoils  his  argument  based  on  similarity.  The 
third  argument  in  favor  of  the  Cambrian  age  of  the  Animikie,  based  on  the 
extent  of  the  unconformity  beneath  the  Animikie,  has  little  value  when 
unsupported  by  the  other  lines  of  evidence.  Professor  Winchell's  conclusion 
as  to  the  Cambrian  age  of  the  Animikie  strata  is  thus  not  adequately 
sustained  by  the  reasons  given.  The  view  that  the  Animikie  is  Upper 
Huroniau  (pre-Cambrian)  is  the  commonly  accepted  one.  The  evidence 
favoring  this  view  is  summarized  by  Van  Hise." 

Further  comment  on  the  above  work  would  require  reference  to  the 
detailed  observations  made  in  northeastern  Minnesota  during  four  years  by 
the  Lake  Superior  Division  of  the  United  States  Geological  Survey.  The 
results  of  this  work  are  published  in  this  monograph.  In  general  it  may 
be  stated  that  now,  as  in  the  past,  there  is  a  divergence  in  the  conclusions 
reached  by  the  Minnesota  survej^  and  by  the  United  States  Greological 
Survey  concerning  the  position  and  importance  of  the  unconformities,  the 
correlation  of  series,  and  the  nomenclature. 

1900. 

Coleman,  A.  P.     Copper  and  iron  regions  of  Ontario;  with  Report  of  the  Ontario 
Bureau  of  Mines,  1900,  pp.  143-191. 

This  paper  deals  incidentally  with  the  Vermilion  district  (pp.  150-154). 
Before  the  regular  field  work  was  begun  in  Ontario  the  author,  accom- 
panied by  Prof.  Arthur  B.  Willmott,  visited  the  Lake  Superior  iron  ranges 
of  the  United  States.  The  Vermilion  range  was  visited,  among  others,  and 
a  few  desultory  observations  concerning  the  mines  and  rocks  in  the  vicinity 
of  the  mines  are  recorded. 

Grant,  U.  S.     Contact  metamorphism  of  a  basic  igneous  rock:  Bull.  Geol.  Soc. 
Am.,  Vol.  II,  1900,  pp.  503-510. 

Along  the  northern  edge  of  the  Great  gabbro  mass  of  northeastern 
Minnesota  there  occur  certain  peculiar  crystalline  rocks.  These  have  been 
produced  by  the  contact  action  of  the  gabbro  on  rocks  of  varied  lithologic 

"Correlation  Papers,  Archean  and  Algonkian,  Bull.  U.  S.  Geol.  Survey  No.  86;  Principles  of  pre- 
Cambrian  geology :  Sixteenth  Ann.  Kept.  V.  S.  Geol.  Survey,  Pt,  I,  1896,  pp.  571-874. 


126  THE  VERMILION  IRON-BEARING  DISTRICT. 

character.  It  is  the  object  of  the  paper  to  give  in  outline  an  account  of  the 
phenomena  seen  on  these  rocks.  An  outline  of  the  local  geology  is  given. 
The  rocks  range  from  Archean,  through  the  Lower  Huronian  (or  Keewatin) 
and  Upper  Huronian  (or  Animikie),  to  the  Keweenawan.  The  Animikie  of 
this  area  is  subdivided  from  the  base  up  into  an  iron-bearing  member,  a  black- 
slate  member,  and  a  graywacke-slate  member.  The  Keweenawan  is  rep- 
resented by  the  gabbro,  which  has  metamorphosed  the  Gunflint  iron-bearing 
beds,  and  the  graywacke  and  black  slate  members  of  Upper  Huronian  age. 

The  metamorphism  of  the  black-slate  and  graywacke-slate  members  is 
relatively  insignificant,  and  consists  of  the  more  or  less  complete  recrvstal- 
lization  of  the  rocks,  which  are  now  made  up  of  granitic  aggregates  of 
quartz,  feldspar,  biotite,  muscovite,  and  occasionally  cordierite. 

The  most  complete  recrystallization  and  the  most  interesting'  phe- 
nomena are  shown  in  the  metamorphosed  iron-bearing  beds. 

The  original  rock  is  regarded  as  a  glauconitic  greensand  in  which  there 
is  more  or  less  iron  carbonate.  This  has  been  altered  to  a  quartz-magnetite- 
amphibole-slate,  the  amphibole  being  in  the  form  of  actinolite,  griinerite, 
cummingtonite,  and  hornblende.  This  has  been  profoundly  changed  by 
the  gabbro,  and  is  now  a  coarse-grained  aggregate  of  quartz,  magnetite, 
olivine  (which  is  frequently  fayalite),  hypersthene,  augite,  hornblende,  and 
occasionally  griinerite  and  cummingtonite.  These  rocks,  like  the  rocks 
from  which  they  are  derived,  are  beautifully  banded,  the  separate  bands 
being  composed  of  quartz,  or  of  magnetite,  or  of  silicates,  or  of  a  mixture  of 
any  two  or  more  of  the  minerals. 

Satisfactory  reasons  are  given  showing  that  these  rocks  are  a  part 
of  the  Animikie  (Upper  Huronian)  and  not  a  border  facies  of  the  gabbro,  as 
has  been  thought  to  be  the  case  by  some. 

The  gabbro  is  in  contact  with  very  diverse  strata  of  the  Keewatin,  and 
the  resulting  metamorphic  rocks  differ  greatly.  Biotite  and  hypersthene 
are  prominent  in  these  contact  rocks. 

The  Archean  consists  of  granites  and  greenstones.  The  granites  have 
not  been  affected  by  the  gabbro  in  a  noticeable  way  at  least.  The  green- 
stones have  been  affected  in  such  a  way  as  to  reproduce  the  minerals 
of  the  original  rocks  from  which  the  greenstones  were  derived.  Com- 
monly a  granular  product  is  found  which  is  quite  similar  in  appearance  to 
a  gabbro. 


RESUME  OF  LITERATURE.        •  127 

Comments. — The  members  of  the  United  States  Geological  Sm-vey  who 
have  been  studying  this  area  place  the  dividing  lines  in  some  cases  at  places 
somewhat  different  from  those  where  Grant  places  them.  They  make  the 
same  main  divisions,  however.  The  glauconitic  greensand  of  Grant  is  now 
known  not  to  contain  potassium,  the  green  granules  being  composed  of  a 
hydrous  ferrous  silicate. " 

The  United  States  geologists  concur  in  the  general  conclusions  reached 
by  Grant  as  to  the  character  and  cause  of  the  metamorphism. 

1901. 

WiNCHELL,  N.  H.  The  geology  of  Minnesota:  Final  Report  Geol.  and  Nat. 
Hist.  Survey  of  Minn..  Vol.  VI,  1901.  Geological  atlas  with  synoptical  descriptions, 
88  plates. 

This  is  a  collection  of  maps  of  Minnesota.  The  earlier  ones,  from 
Franquelins  map  of  1688  up  to  and  including  Nicollet's  of  1842,  are  repro- 
duced, and  then  the  later  maps  published  by  the  Geological  and  Natural 
History  Survey  in  the  reports  preceding-  this.  Brief  explanatory  notes 
accompany  each  plate. 

WiNCHELL,  N.  H.  Glacial  lakes  of  Minnesota:  Bull.  Geol.  Soc.  Am..  Vol.  XII, 
1901,  pp.  109-128,  pi.  12. 

Winchell  gives  a  brief  description  of  a  number  of  glacial  lakes  occur- 
ring in  Minnesota.  Among  these  there  are  two,  Lake  Norwood  and  Lake 
Onnamani,  which  lie  partially  or  wholly  in  the  Vermilion  district. 

Van  Hise,  C.  R.  The  iron-ore  deposits  of  the  Lake  Superior  region:  Twenty- 
first  Ann.  Rept.  U.  S.  Geol.  Survey,  Pt.  Ill,  1901,  pp.  305-434,  12  plates,  6  of 
which  are  geological  maps. 

This  paper  contains  a  brief  description  and  comparison  of  the  various 
iron-bearing  districts  of  the  Lake  Superior  region. 

The  first  chapter  contains  a  general  discussion  of  jwhiciples.  The 
rocks  of  the  region,  disregarding  the  late  formations,  are  stated  to  be  divisible 
into  the  following  five  series,  enumerated  from  base  up :  Archean,  Lower 
Huronian,  Upper  Huronian,  Keweenawan,  and  Cambrian. 

The  chief  varieties  of  the  iron-bearing  rocks  and  their  alterations  are 
described.     They  are  shown  to  occur  in  the  three  series,  the  Archean,  Lower 

«The  Mesabi  iron-bearing  district  of  Minnesota,  by  C.  K.  Leith:  Mon.  U.  S.  Geol.  Survey  Vol. 
XLIII,  1903,  pp.  110. 


128  THE  VERMILION  IRON-BEARING  DISTRICT. 

Hurouian,  and  Upper  Hurouiaii.  The  genesis  of  the  ore  deposits  is  then 
discussed. 

The  genei'al  process  of  ore  formation  for  the  Lake  Superior  region  as 
a  whole  is  the  same  as  that  ah'eady  described  in  the  monographs  on  the 
Penokee  and  Marquette  districts.  The  iron  is  leached  from  an  older  forma- 
tion—  in  case  of  the  Archean  from  igneous  rocks,  the  greenstones — and  is 
then  deposited  as  a  sedimentary  formation,  in  the  form  of  a  cherty  carbonate 
or  some  iron  compound,  which,  however,  becomes  this  cherty  carbonate. 
These  formations  are  folded  and  intruded  by  igneous  rocks,  and  pitching 
troughs  with  relatively  impervious  sides  and  bottom  are  formed.  Meteoric 
waters  carry  downward  in  solution  iron  derived  from  the  iron-bearing  for- 
mation, and  this  is  precipitated  as  an  oxide  in  the  structural  basins  in  the 
formation  and  in  the  spaces  left  by  removal  of  the  silica.  As  result  of  this 
replacement,  enrichment  of  the  iron-bearing'  formation  occurs  at  favorable 
places,  and  the  ore  deposits  are  formed. 

Sections  are  devoted  to  a  consideration  of  the  influence  of  topography 
and  denudation  on  ore  deposition,  and  to  a  discussion  of  the  time  and 
depth  of  concentration  of  the  ore  deposits. 

In  Chapter  II  the  individual  districts  are  taken  up.  The  section  devoted 
to  the  Vermilion  district  is  written  by  Van  Hise  and  Clements,  and  in  this 
is  given  the  first  detailed  statement  made  by  the  United  States  Geological 
Survey  on  this  district.  A  preliminary  map  accompanies  the  description. 
Since  this  section  on  the  Vermilion  district  is  in  fact  a  very  brief  abstract 
of  the  present  monograph,  it  will  not  be  reviewed  in  detail.  It  may  be  well 
to  mention  the  fact  that  the  Archean  is  here  made  to  include  certain  sedi- 
mentary rocks  In  the  last  chapter,  the  third,  there  is  a  comparison  and 
summary  of  all  the  districts.  The  Vermilion  district  is  again  briefly  con- 
sidered, and  some  suggestions  are  offered  in  regard  to  exploration  in  it. 


CHAPTER    III. 

THE  ARCHEAN. 

SECTION  I— DEFINITION  AND  SUBDIVISIONS, 

The  Archean,  as  heretofore  defined  by  Professor  Van  Hise,  was  made 
to  indude  all  pre-Algonkian  rocks,  and  these  were  supposed  to  be  igneous 
rocks  only."  As  a  result  of  the  work  of  the  field  season  of  1900  on  the 
north  shore  of  Lake  Superior  in  Minnesota  and  Canada,  it  has  been  found 
necessary  to  modify  our  ideas  of  the  Archean  and  to  change  the  definition 
of  the  word  accordingly.  The  term  Archean,  as  used  in  the  present  paper, 
comprises  rocks  older  than  the  Algonkian,  whicli  are  predominantly  of 
igneous  origin,  but  with  which  may  be  included  some  subordinate  amounts 
of  sediments. 

From  the  study  of  the  Vermilion  district  it  has  been  found  possible  to 
divide  the  Archean  of  that  district  into  three  stratigraphic  divisions,  which, 
enumerated  in  order  of  age,  beginning  with  the  lowest,  are:  Ely  greenstone, 
Soudan  formation,  and  a  series  of  granitic  rocks.  The  Ely  greenstone 
consists  of  basic  to  intermediate  igneous  rocks,  and  is  the  lowest  member 
of  the  geologic  column  in  this  district.  Above  this  occtirs  an  iron-bearing 
formation,  the  Soudan  formation,  of  totally  different  lithologic  character 
and  mode  of  origin,  whose  base  is  marked  here  and  there  by  a  conglom- 
erate of  small  extent.  The  iron-bearing  formation  is  followed  by  a  series 
of  acid  intrusives  varying  from  rhyolite-porphyries  to  granites  and  granite- 
porphyries,  with  normal  granites  as  the  predominant  form.  These  rocks 
show  in  many  places  their  intrusive  relationship  to  both  of  the  earlier 
formations.     These  three  formations  constitute  the  Archean  and  are  sepa- 

« Correlation  papers — Archean  and  Algonkian:  Bull.  D".  S.  Geol.  Survey  No.  86, 1892,  pp.  156-199. 
Also  Principles  of  North  American  pre-Cambrian  geology,  by  C.  R.  Van  Hise:  Sixteenth  Ann.  Rept. 
U.  S.  Geol.  Survey,  Pt.  I,  1896,  pp.  581-872. 

MON  XLV — 03 9  129 


130  THE  VERMILION  IRON-BEARING  DISTRICT. 

rated  from  tlie  next  succeeding  series  by  a  great  unconformity.  The 
fomiations  will  be  taken  up  in  order  of  age  and  described  separately  in  the 
following  pages. 

SECTION  II— ELY  GREENSTONE, 

FEATURES  OF  THE  GREEXSTO^TE. 

In  the  Vermilion  district  there  is  a  great  complex  of  igneous  rocks 
whose  members  possess  one  general  character  in  common.  They  are 
almost  universally  colored  some  shade  of  green ;  hence,  for  want  of  a  better 
one,  the  descriptive  term  "greenstone"  is  applied  to  the  complex.  Tojudge 
from  microscopic  examination,  no  chemical  analyses  having  been  made, 
the  rocks  constituting  this  complex  vary  in  chemical  character  from  inter- 
mediate to  basic  rocks.  They  likewise  possess  varied  physical  characters. 
The  various  parts  of  this  complex  are  not  of  exactly  the  same  age,  as  in  a 
number  of  places  one  member  of  the  complex  intrudes  other  members ; 
nor  are  the  rocks  all  of  exactly  the  same  mode  of  origin,  as  some  are 
effusive  and  others  intrusive,  although  all  are  igneous.  These  differences 
in  age  and  mode  of  formation  are,  however,  only  such  as  we  normally  expect 
to  find  in  rocks  belonging,  as  these  do,  to  one  great  period  of  eruptive 
acti-vdty  which  certainly  extended  over  a  great  length  of  time. 

Finally,  a  number  of  patches  of  fragmental  rock  are  found  in  the  midst 
of  the  Ely  greenstones.  These  patches  are  too  small  to  be  shown  on  the 
accompan^-ing  maps.  Moreover,  it  is  still  doubtful  whether  these  are  tuffs 
contemporaneous  with  and  interbedded  v>hh  the  flows,  or  nonnal  conglom- 
erates derived  from  the  greenstones  and  infolded  in  them.  It  is  probable 
that  both  of  these  rocks  are  present,  but  owing  to  the  imperfect  exposures 
it  was  impossible  to  distinguish  them. 

As  a  unit  the  Ely  greenstones  bear  the  following  general  relations  to 
the  rest  of  the  rocks  of  the  district.  The}'  are  in  all  cases  older  than  the 
other  rocks.  From  them  have  been  largely  derived  the  later  elastics  and 
upon  them  rest  all  of  the  sediments.  Througli  them  have  been  intruded 
all  of  the  remaining  igneous  rocks.  The  Ely  greenstones  are  all 
Archean,  or,  if  a  more  general  term  is  desired,  they  form  the  basement 
complex  of  the  Vermilion  district.  This  complex  is  very  well  developed  m 
the  immediate  vicinity  of  Ely,  the  largest  city  of  the  district,  which  is 
literally  built  upon  a  firm  foundation  of  tliese  greenstones,  and  the  complex 


s 


ELY  GREENSTONE.  131 

shows  some  of  its  most  ty|3ical  and  interesting'  characters  right  in  the  streets 
and  lots  of  the  city.  For  this  reason  the  formation  has  been  appropriately 
called  the  "Ely  greenstone."" 

OCCURRENCE  AKD  CHARACTER. 

DISTRIBUTION. 

In  the  westernmost  part  of  the  district,  in  the  vicinity  of  Vermilion 
Lake,  the  area  underlain  by  the  greenstones  has  the  form  of  a  nmnber  of 
large  westward-projecting  tong'ues.  Beginning  their  enumeration  from 
south  to  north  we  find  the  first  large  tongue  extending  through  the  northern 
portion  of  T.  61  N.,  R.  15  W.  North  of  this  there  is  another  tongue,  in 
sees.  35  and  36,  T.  62  N.,  R.  15  W*.  This  is  followed  to  the  north  by  a  third 
tongue,  in  sees.  24,  25,  and  26,  of  the  same  township  and  range.  A  number 
of  very  small  tongues  are  to  be  found  in  the  northern  portion  of  sec.  21,  T.  62 
N.,  R.  14  W.  A  very  narrow  greenstone  belt  extends  to  the  southern 
portion  of  sees.  15,  16,  17,  and  18,  T.  62  N.,  R.  14  W.  Still  another 
such  tongue  is  in  sec.  12,  T.  62  N.,  R.  15  W.,  and  extends,  except  in 
one  small  area,  eastward  through  sees.  7,  8,  and  9,  T.  62  N.,  R.  15  W. 
Still  farther  north  we  find  a  tongue  south  of  Bass  Lake,  in  sees.  1,  2,  3,  and 
4,  T.  62  N.,  R.  15  W.,  and  one  immediately  north  of  this,  just  along  the 
line  between  T.  62  N.,  and  T.  63  N.  The  northern  side  of  Vermilion  Lake 
is  bordered  by  this  Archean  greenstone,  which  has  been  followed  out  to  the 
west  to  the  limit  of  the  area  mapped.  The  greenstone  extends  a  long  dis- 
tance to  the  west  of  the  Vermilion  district,  although  it  is  discontinuous  over 
great  areas.  While  it  may  be  that  this  interruption  of  the  continuity  of  the 
greenstone  in  this  portion  of  the  State  is  due  to  its  concealment  in  places  by 
overlying  drift,  it  is  also  highly  probable  that  even  were  this  drift  removed 
we  should  find  that  the  continuation  of  the  greenstone  is  interrupted,  as  it 
is  in  the  vicinity  of  Vermilion  Lake,  by  the  overlapping  of  the  younger 
formations. 

In  all  cases  these  tongues,  when  followed  otit  to  the  east,  unite  with 
the  main  mass  of  the  Archean  which,  along  the  line  between  Rs.  13  and 

«The  term  "Kawishiwin"  has  been  proposed  by  the  Minnesota  survey  (Geol.  and  Nat.  Hist. 
Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  pp.  270-271  and  546)  to  comprise  the  two  formations 
which  in  this  volume  are  treated  under  the  terms  the  "Ely  greenstone"  and  the  "Soudan  formation," 
as  well  as  certain  other  rocks.  Since  this  throws  together  two  important  formations  that  are  here 
treated  separately,  it  has  seemed  necessary  to  introduce  new  names  for  each  of  these. 


132  THE  VERMILION  IRON-BEARING  DISTRICT. 

14  W.,  covers  almost  the  entire  central  portion  of  the  district  and  has  there 
an  approximate  width  of  10  miles.  This  great  width  of  the  Ely  green- 
stone continues  to  the  east  very  nearly  as  far  as  Pine  Lake,  in  R.  10  W. 
Within  this  area  its  continuity  is  interrupted  in  a  number  of  places  by  nar- 
vow  belts  of  sediments  of  later  age  trending  approximately  east  to  east- 
northeast.  These,  altliough  of  minor  importance  so  far  as  areal  distribu- 
tion is  concerned,  are  of  great  economic  importance,  as  within  the  area 
just  described  they  consist  for  the  most  part  of  the  iron-bearing  formation. 
About  7  miles  west  of  Ely,  in  sec.  4,  T.  62  N.,  R.  13  W.,  begins  a  series 
of  sedimentary  rocks  which  splits  the  Ely  greenstone  approximately  in 
the  center.  This  belt  of  sedimentary  rocks  continues  on  to  the  east  with 
approximately  uniform  width  beyond  the  end  of  Fall  Lake,  in  sec.  31, 
T.  64  N.,  R.  10  W.,  where  it  ends.  Just  about  three-quarters  of  a  mile 
beyond  this  there  begins  another  belt  of  sediments  which  continues 
eastward,  widening  very  rapidly,  and  corresponding  to  this  widening 
there  is  a  rapid  reduction  in  the  width  of  the  greenstone  areas  lying 
north  and  south  of  it.  The  northern  belt  of  greenstone  narrows  more 
rapidly  and  disappears  north  of  Moose  Lake,  in  sec.  16,  T.  64  N.,  R.  9  W., 
where  it  is  covered  by  overlapping  sediments.  The  continuation  of  this 
belt  is  wanting  for  a  distance  of  about  a  mile.  It  then  begins  again  in  sec. 
10,  T.  64  N.,  R.  9  W.,  and  continues  thence  northeast  to  the  international 
boundary. 

From  here  on  to  the  northeast  the  extension  of  the  Ely  greenstone 
has  been  traced  into  Canada  as  the  result  of  a  reconnaissance  survey.  The 
boundaries  of  the  formation  in  Canada,  as  given  upon  the  accompanying- 
maps,  can  not,  therefore,  be  regarded  as  nearly  so  correct  as  those  given 
witliin  the  United  States.  They  have  been  found  in  most  places  as  a  result 
of  the  study  of  exposures  along  the  lakes  and  of  a  few  traverses  inland. 
This  reconnaissance  shows  us  that  the  width  of  the  Ely  greenstone,  as  it 
continues  to  the  northeast,  varies  greatly  as  a  result  of  the  folding  to  which 
it  has  been  subjected.  As  a  result  of  this,  also,  its  continuity  is  interrupted 
by  the  infolding  of  the  younger  rocks  whose  are.cS  have  been  delimited. 
Upon  the  map  it  appears  to  cover  a  larger  area  than  it  does  in  reality,  for 
the  reason  that  the  important  iron-bearing  formation  has  not  been  delimited, 
although  its  presence  within  the  greenstone  area  is  known  from  its  having 
been  observed  at  a  great  number  of  j)lnces. 


ELY  GREENSTONE.  133 

Returning  to  the  sovitbern  arm  of  the  greenstone,  we  find  it  covering 
the  major  portion  of  T.  63  N.,  R.  10  W.,  with  a  small  portion  of  T.  64  N. 
As  we  follow  it  eastward  into  T.  63  and  T.  64  N.,  R.  9  W.,  we  find  that  its 
width  is  materially  reduced.  This  results  from  the  fact  that  in  this  area  we 
are  upon  a  great  anticline  plunging  to  the  east,  around  which  wrap  the 
younger  formations.  These  are  likewise  infolded  in  synclines  within  the 
greenstone,  as  is  to  be  expected,  for  instance,  in  sec.  17,  T.  63  N.,  R.  9  W. 
In  the  southern  portion  of  T.  64  N.,  R.  9  W.,  this  infolding  is  beautifully 
shown.  As  a  result  of  this  the  greenstone  is  divided  into  a  number  of 
nan-ow  belts  having,  in  general,  an  east-west  trend,  each  belt  being- 
separated  from  every  other,  and  from  the  main  mass  of  the  greenstone  to 
the  south,  b}^  a  trough  containing  later  sedimentary  deposits.  As  results 
of  cross  folding  the  greenstone  occurs  in  a  number  of  places  in  anticlinal 
boss-like  areas  plunging  down  under  the  sediments  and  completely 
surrounded  by  them. 

From  the  center  of  R.  9  W.  eastwai-d  this  greenstone  is  totally  wanting 
until  we  reach  the  center  of  R.  8  W.,  where  another  anticlinal  area  of 
greenstone  is  found.  This  is  surrounded  on  three  sides  by  overlapping 
sedimentaries,  the  fourth,  the  eastern  side,  being  cut  off  by  the  gabbro. 
As  we  go  farther  east  we  find  that  the  greenstone  is  not  continuous  over 
any  very  broad  areas.  It  occurs  for  the  most  part  in  rather  narrow,  long 
belts;  for  instance,  such  a  belt  begins  on  the  point  projecting  westward 
into  Knife  Lake  in  sec.  21,  T.  65  N.,  R.  7  W.,  and  extends  thence  east- 
ward into  sec.  11,  T.  66  N.,  R.  6  W.,  with  a  maximum  width  of  about 
one-half  mile.  Alongside  this  belt,  however,  there  are  small  isolated 
bosses  surrounded  by  the  younger  rocks,  as  may  be  seen  in  the  southwest 
quarter  of  sec.  17  and  the  southeast  quarter  of  sec.  18,  T.  65  N.,  R.  6  W. 
Similar  greenstone  areas  occur  south  of  Knife  Lake  in  sees.  29,  30,  and 
31,  T.  65  N.,  R.  6  W.,  and  in  sees.  25  and  36,  T.  65  N.,  R.  7  W.,  and  other 
small  areas  occur  also  in  sees.  25,  26,  and  27,  T.  65  N,  R.  6  W.  Consid- 
erably larger  is  the  greenstone  massive  forming  the  ridge  upon  which  the 
Twin  Peaks  are  prominent  points,  extending  along  the  line  between  Ts. 
64  and  65  N.  eastward  to  Gobbemichigamma  Lake.  Still  larger  is  the 
area  that  extends  over  sees.  18  and  19,  T.  65  N.,  R.  6  W.,  eastward  to 
sec.  27,  T.  65  N.,  R.  4  W.,  having  an  east-west  length  of  approximately 
10  miles.     This  last  belt  starts  in  at  the  west  with  two  westward-plunging 


134  THE  VERMILION  IRON-BEARING  DISTRICT. 

anticlinal  tongues  separated  by  sedimentary  formations,  and,  after  varying 
materially  in  width  as  the  result  of  cross  folding,  finally  ends  at  its  eastern 
extremity  also  in  two  tongues — eastward-plunging  anticlines  partially 
surrounded  by  the  younger  sediments. 

EXPOSURES. 

The  exposures  of  greenstone  in  the  areas  outlined  above  are  very  good. 
It  is  no  uncommon  thing  to  find  almost  absolutely  bare  surfaces  several 
hundred  feet  long  and  possibly  one-fourth  as  wide.  Such  exposures  are 
most  commonly  rounded  surfaces.  Occasionally,  however,  cliffs  of  green- 
stone are  seen.  In  spite  of  the  large  size  and  the  great  number  of  the 
exposures,  it  was  very  difficult — in  fact,  in  most  places  almost  impossible — 
to  determine  the  relations  of  the  different  kinds  of  greenstone  to  one  another, 
for  the  contacts  have  usually  been  concealed  either  by  di'ift  or  by  the 
effects  of  erosion,  so  that  where  most  needed,  as  is  commonly  the  case,  the 
exposures  are  wanting.  Mention  will  be  made  later  of  a  few  places  where 
some  of  the  best  exposures  were  found. 

TOPOGRAPHY. 

In  the  Avestern  portion  of  the  district,  where  the  the  greenstone  under- 
lies broad  areas,  the  topography  is  ver}^  much  broken.  The  minor  idges 
are  numerovis  and  form  the  most  prominent  feature.  In  this  portion  of  the 
district  there  is  a  series  of  parallel  ridges  with  narrow  valleys  between 
them.  Usually  the  sides  have  a  steep  slope,  and  there  are  sometimes  abrupt 
escarpments,  but  as  a  rule  the  hills  and  ridges  are  well  rounded.  It  will 
thus  be  seen  that  in  detail  the  topography  is  very  rugged.  Especially  is 
this  so  north  of  Fall  and  Long  lakes.  The  ridges  throughout  the  green- 
stone area  lie  in  essentially  parallel  chains  extending  east-northeast,  a 
direction  corresponding  to  the  trend  of  the  structure  of  the  district.  It  has 
already  been  stated  that  the  Ely  greenstone  occurs  very  rarely  in  broad 
areas  in  the  eastern  portion  of  the  district,  being  there  usually  found  in 
comparatively  small  areas  surrounded  by  younger  sediments.  There  is  a 
very  noticeable  difference  in  the  topography  of  areas  underlain  by  the 
greenstone  and  those  underlain  by  the  surrounding  sediments,  due  to 
differential  erosion.  As  a  rule,  the  greenstone  forms  the  prominent  hills 
and  main  ridges.  Usually,  in  traversing  the  country,  one  finds  that  after 
leaving  the  sediments,  which   lie  within  a  topographic  depression,   there 


ELY  GREENSTONE.  135 

follow  the  low,  rounded  knobs  or  ridges  of  the  greenstone.  After  passing 
over  these  and  ascending  a  gentle  slope  the  top  is  reached,  which  is  iisually 
a  broad,  flat  dome.  The  descent  on  the  other  side  carries  one  over  similar 
topography,  with  the  topographic  break  intervening  in  most  cases  between 
the  greenstone  and  the  sediments.  In  these  large  ridges  the  minor  details 
of  the  topography  are  usually  not  very  strongly  accentuated,  but  in  each 
case  blend  in  the  main  ridge  which,  while  forming  a  very  marked  topographic 
feature,  is  in  general  not  separated  into  distinct  peaks. 

STRUCTURE. 

In  view  of  the  essentially  homogeneous,  igneous  character  of  thfe  Ely 
greenstone,  it  will  be  readil}^  seen  that  the  geologic  structure  of  the  green- 
stones areas  could  not  have  been  determined  without  the  aid  of  the  younger 
sedimentary  formations.  As  the  result  of  the  study  of  the  district,  we 
find  that  the  greenstones  have  been  intricately  folded,  the  folds  have  in  many 
instances  been  carefully  traced,  and  it  has  been  found  that  in  general  the 
greenstone  has  been  folded  into  a  great  syuclinorium.  The  character  of 
this  is  better  brought  out  in  the  western  than  in  tlie  eastern  part  of 
the  district.  Within  this  synclinorium  the  synclines  are  occupied  by  the 
younger  rocks,  whereas  the  anticlines  are  of  greenstone  projecting  throug'h 
sediments  of  younger  age.  Typical  anticlines  of  the  greenstone,  partially  sur- 
rounded by  the  sedimentaries,  occur  in  the  vicinity  of  Vermilion  Lake,  in  the 
western  part  of  the  district  and  are  enumerated  on  page  432.  Attention  is  here 
again  called  to  the  possibility  that  the  greenstones  reported  to  occur  west  of 
that  part  of  the  district  mapped  are  perhaps  the  crest  of  greenstone  anticlines 
projecting  through  the  drift.  The  rocks  of  the  Vermilion  district  have  been 
affected  by  a  second  system  of  folds  lying  approximately  at  right  angles  to 
those  that  form  the  great  east-west  trending  synclinorium.  The  effect  of 
this  cross  folding  is  best  shown  by  the  steeply  plunging  anticlines  and 
synclines  in  the  sediments  of  later  age,  as  well  as  by  the  distribution  of  the 
formations  in  general.  If  we  examine  the  map  including  the  area  near  the 
west  end  of  Moose  Lake,  we  find  that  as  a  result  of  the  main  folding  the 
gi'eenstone  has  been  divided  into  a  number  of  narrow  belts  separated  from 
one  another  north  and  south  by  still  narrower  belts  of  sediments  lying  in 
synclines  between  the  greenstones.  It  will  be  noted,  also,  that  some  of  these 
belts  are  completely  isolated  and  that  others  have  but  slight  connection. 


136  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  isolation  of  the  belts  is  due  to  the  effect  of  the  cross  folding  which  has 
produced  anticlines  of  greenstone  plunging  down  iinder  sediments  that 
wrap  around  them.  The  most  striking  eases  of  these  isolated  anticlines  are 
those  shown  by  the  distribution  of  the  greenstone  in  the  vicinity  of  Knife 
and  Cacaquabic  lakes  and  between  Ogishke  Muucie  and  Grobbemichigamma 
lakes.  Looking  at  the  distribution  of  the  greenstone  proper,  we  see  that 
the  presence  of  the  synclinal  structure  is  most  marked  in  the  western  part 
of  the  district  where  the  younger  formations  are  infolded  into  the  greenstone 
and  where  the  greenstone  predominates.  In  the  eastern  part  of  the  district, 
on  the  other  hand,  the  anticlinal  structure  of  the  greenstone  is  most  marked 
for  the  reason  that  the  minor  synclines  are  very  deeply  buried  by  the 
sedimentaries,  which  have  a  great  surficial  extent,  as  a  result  of  which  only 
the  crests  of  the  anticlines  are  exposed  where  they  project  through  the 
sedimentaries. 

PETROGRAPHIC  CHARACTERS. 

The  rocks  comprised  in  the  Ely  greenstone  originally  corresponded  in 
character  to  intermediate  andesites  and  basic  basalts  and,  like  the  recent 
representatives  of  these  families,  must  have  been  black  or  dark  gray,  and 
presumably  likewise  corresponded  to  them  in  mineralogic  character.  These 
Archean  rocks  have  undergone  for  so  long  a  period  the  vicissitudes  to 
which  all  rocks  are  exposed  that  it  is  not  to  be  wondered  at  that  they  have 
for  the  most  part  been  exceedingly  altered  and  never  show  all  of  their 
original  characters.  Indeed,  it  is  surprising  that  they  retain  any  of  their 
original  structures.  Most  of  the  changes  which  have  affected  them  have 
been  in  the  character  of  the  minerals,  and  these  will  be  described  in  the 
proper  place.  The  changes  that  are  most  obvious  macroscopically  are 
the  chemical  ones  which  have  affected  their  color  and  the  mechanical  ones 
which  have  affected  their  structure. 

While  the  rocks,  on  the  whole,  are  of  a  greenish  color .  (lience  the 
general  name  of  greenstone),  the  various  phases  of  them  show  all  possible 
variants  of  this,  ranging  from  very  light-colored  greenish  gray  to  very 
dark  greenish  black,  with  the  light  grayish  and  brownish  greens 
predominating.  The  rocks  appear  lighter  on  the  weathered  surface,  as  a 
rule,  than  upon  fresh  fracture  surfaces.  Some  of  the  greenstones  are  very 
nnxcli  more  feldspathic  than  usual,  and  in  such  cases  they  weather  with  a 
light-pinkish   crust,  which  causes  them  not  uncommonly  to  be  mistaken, 


ELY  GREENSTONE.  137 

when  viewed  from  a  distance,  for  the  more  acid  granites.  The  macroscopic 
textures  commonly  seen  are  the  ophitic,  the  poikilitic,  and  the  porphyritic. 
The  rocks  possessing  these  textures  vary  from  fine-grained,  almost  aphanitic, 
ones  to  those  which  are  very  coarse  grained  and,  in  exceptional  cases, 
have  some  constituents  an  inch  and  a  half  in  length.  The  porphyi'itic 
rocks  have  as  phenocrysts  feldspar  or  hornblende,  or  both,  in  a  matrix 
w'hich  vai'ies  from  fine  to  coarse  in  grain.  Some  of  the  feldspar  phenocrysts 
are  an  inch  and  a  half  in  length.  Many  of  the  finer  grained  forms  of  these 
rocks  are  amygdaloidal  and  also  frequently  show  beautiful  cases  of 
spherulitic  development. 

Good  columnar  parting  is  totally  wanting  in  the  greenstones  of  the 
Vermilion  district,  but,  apparently  taking  its  place,  ellipsoidal  parting"  is 
abundantly  present.  All  combinations  of  the  above  structures  and  textures 
may  be  found  in  this  complex  and  all  gradations  between  the  rocks 
possessing  them.  Thus  we  find  gradations  from  fine  to  coarse  forms 
and  from  the  nonporphyritic  to  the  porphyritic.  Those  that  are  not 
amygdaloidal  at  one  place  may  become  amygdaloidal  elsewhere,  and  with 
this  change  we  may  find  the  rocks  becoming  porphyritic,  possibly  showing 
ellipsoidal  parting.  Fine-grained  ellipsoidal  and  sjDherulitic  basalts  grade 
into  coarse-grained  ellipsoidal  spherulitic  basalts,  or  into  coarse-grained 
basalts  that  are  neither  ellipsoidal  nor  spherulitic. 

The  greenstones  are  predominantly  massive.  Nevertheless  they  show 
the  effect  of  dynamic  action  and  are  in  many  cases  finely  jointed.  The 
dynamic  action  aifecting  them  has  resulted  in  the  production  in  several 
places  of  very  excellent  friction  breccias  (reibungs-breccias)  which  can 
with  difficulty  be  separated  from  tuffaceous  or  conglomeratic  deposits.  On 
the  bare  hills  south  of  Moose  Lake  these  basalts  are  in  places  brecciated, 
producing  rocks  that  strikingly  simulate  greenstone  conglomerates.  In 
most  cases  the  brecciated  zone  has  a  width  of  only  a  few  feet.  These 
breccias  might  readily  be  mistaken  for  true  conglomerates  if  the  adjacent 
massive  rocks  were  covered  and  the  breccias  only  were  exposed.  The 
ellipsoids  on  great  numbers  of  the  exposures  have  very  numerous  and 
prominent  gashes  w^hich  traverse  them  at  vai'ious  angles,  though  usually 
nearly  at  right  angles  to  the  direction  of  elongation.  These  are  clearly 
indicative  of  the  mashing  to  which  these  ellipsoids  have  been  subjected. 

"Mou.  U.  S.  Geol.  Survey  Vol.  XXXVI,  1899,  pp.  112-124. 


138  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  masliing  of  the  Ely  greenstone  has  resulted  in  producing  very  com- 
monly an  imperfect  schistose  structure  in  tlie  rocks  composing  it.  In 
exceptional  cases  and  locally  this  development  of  schistosity  has  advanced 
to  such  a  point  that  the  rocks  have  become  completely  schistose,  and  even 
finely  fissile.  Gradations  from  the  massive,  coarse-grained  greenstones  to 
green  schists  of  dynamic  origin  have  been  observed  in  a  nmnber  of  places. 
This  schistose  structure  has,  however,  certainly  not  reached  such  universal 
development  that  one  would  be  warranted  in  speaking  of  the  Ely  green- 
stone as  a  green  schist  complex. 

Finally,  there  occurs  in  a  number  of  places  with  the  rocks  described  a 
conglomerate  or  tuff,  whose  structural  relations  to  the  greenstones  are 
somewhat  doubtful.  This  facies  is  found  commonly  in  small  and  isolated 
exposures  or  under  other  conditions  that  preclude  the  determination  of 
its  relationship  to  the  nearest  greenstones.  These  rocks  consist  of  more 
or  less  rounded  but  irregularly  shaped  fragments  of  greenstones  of 
various  kinds,  but  corresponding  to  those  that  occur  all  around  them 
in  the  district.  One  can  not  say,  however,  that  these  deposits  consist 
chiefly  of  fragments  of  greenstones  like  those  that  are  nearest  them.  No 
definite  indications  of  bedding  have  in  any  case  been  found,  nor  do  the 
rocks  occur  in  connection  with  any  sediments  of  which  they  can  be  the  basal 
conglomerates.  In  some  places  they  lie  between  exposures  of  the  massive 
greenstone,  and  one  is  inclined  to  interpret  such  a  field  relationship  as  due 
to  alternation  of  flows  and  tuff  deposits.  On  the  other  hand,  however,  one 
m&j  readily  interpret  this  relation  as  due  to  infolding  of  the  elastics  in  the 
greenstones.  Rocks  very  similar  to  these,  but  showing  the  transition  to 
finer-grained,  clearly  sedimentary  deposits  have  been  found  in  several 
places,  and  are  descril^ed  Avith  the  Soudan  formation,  to  which  they 
belong.  The  latter  deposits  clearly  owe  their  field  relationship  to  infolding 
within  the  greenstones.  It  is  highly  probable  that  most,  if  not  all, 
of  these  conglomeratic  rocks  should  be  so  classed.  However,  a  few  cases, 
which  will  be  mentioned  under  the  heading  "  Interesting  localities,"  have 
been  doubtfully  referred  as  tuffs  to  the  Ely  greenstone.  These  elastics 
have  nothing  to  do  with  those  belonging  to  the  Ogishke  conglomerate, 
which  will  be  subsequently  coiasidered. 

In  numerous  places  the  altered  greenstones  have  been  more  or  less 
thoroughly  discolored  and  impregnated  with  iron.     This  impregnation  is,  in 


ELY  GREENSTONE.  139 

all  of  the  cases  observed,  almost  purely  superficial,  extending  at  most  only  a 
few  feet  down  into  the  rocks.  Such  occurrences  have  led  to  considerable 
waste  of  money  in  the  sinking  of  prospect  holes.  The  joints  and  g-ashes  in 
numerous  places  throughout  the  district  have  been  found  filled  more  or  less 
completely  with  quartz,  occurring-  both  as  vein  quartz  and  in  a  saccharoidal 
condition,  more  or  less  intimately  mixed  with  carbonates.  In  several  places 
large  veins  of  quartz  traverse  the  greenstones,  but  minute  ones  are  more 
common.  The  largest  veins  have  been  prospected  for  gold,  and  several 
gold  mines,  so  called,  have  been  opened  along  them.  Where  the  quartz 
veins  are  mixed  with  carbonates  the  carbonate  usually  carries  a  consider- 
able content  of  iron,  so  that  on  weathered  surfaces  such  vein  deposits  are 
quite  ferruginous.  This  infiltrated  carbonate-bearing  material"  is  especially 
common  in  the  interstices  and  in  the  schistose  matrix  between  the  ellipsoids. 
In  this  same  position,  and  apparently  but  a  further  alteration  product 
of  such  secondary  carbonate-bearing  deposits,  is  a  white,  black,  or  purplish 
chert,  and  less  frequently  a  red  jasper.  Not  uncommonly,  also,  the  non- 
ellipsoidal  greenstone  near  the  jasper  belts  contains  irregular  bunches  and 
lenticular  areas,  varj'ing  in  size,  of  rather  coarse  white  and  black  chert, 
with  more  rarely  the  true  red  jasper.  Such  deposits  are  certainly,  in  many 
cases,  composed  of  infiltration  products  brought  from  overlying  formations 
and  deposited  in  the  interstices  of  the  basal  greenstones.  They  are  never 
of  very  large  size,  and  it  is  of  course  useless  to  prospect  in  such  places  for 
paying  ore  bodies. 

Certain  of  the  macroscopic  structures,  namely,  the  amygdaloidal,  the 
spherulitic,  and  the  ellipsoidal,  mentioned  above  as  being  present  in  these 
greenstones,  are  very  well  developed,  and  on  account  of  this,  but  chiefly  as 
they  offer  a  clew  to  the  mode  of  formation  of  the  greenstones,  they  are  of 
some  interest  and  will  be  described  m  detail.  ' 

THE    AMYGDALOIDAL    STRUCTURE. 

Of  the  three  structures  mentioned  above  the  most  common  is  the 
amygdaloidal.  This-  is  rarely  seen  in  the  very  coarse  greenstones,  but  is 
usually  present  in  the  finer-grained  varieties.  This  structure  is  most  notice- 
able on  weathered  surfaces,  where  it  may  be  recognized  by  the  presence  of 
rounded  or  oval  spots  scattered  over  the  rock.      On  examining  the  internal 

«  Mon.  U.  S.  Geol.  Survey  Vol.  XXXVI,  1899,  pp.  130-135. 


140  THE  VERMILION  IRON-BEARING  DISTRICT. 

structure  of  the  greenstone,  one  sees  that  these  spots,  which  are  also  scattered 
through  the  body  of  the  rock,  are  frequently  cross  sections  of  irregular  tubes 
lying  perpendicular  to  the  surface  of  the  flow.  They  are  sections  through 
filled  gas  pores,  the  filling  being  known  as  amygdules.  The  amygdules 
consist  of  chlorite,  calcite,  quartz,  actinolite,  and  epidote.  Rarely  is  any 
one  of  the  minerals  absolutely  alone  in  the  amygdule,  though  usually  one  or 
the  other  will  greatly  predominate.  The  amygdules  are  filled  with  chlonte ; 
chlorite  and  quartz;  calcite;  chlorite  and  calcite;  chlorite  and  epidote; 
chlorite  and  actinolite;  chlorite,  calcite,  and  quartz;  calcite  and  quartz;  and 
quartz  alone.  The  materials  constituting  these  amygdules  are  arranged 
above  in  approximately  the  order  of  abundance.  According  to  whether 
the  light  or  the  dark  minerals  predominate,  we  get  light-colored  to  white 
gi-anular  areas  on  the  one  hand,  grading  to  light-greenish  to  silky  dark- 
green  areas  on  the  other.  The  gas  pores  of  these  rocks  owe  their  origin, 
as  has  already  been  hinted  at,  to  the  pressure  of  the  gas  in  an  originally 
molten  magma.  When  this  magma  reached  a  position  where  the  pressure 
was  markedly  diminished,  the  gas  separated,  segregated,  'and  expanded, 
and  the  magma  became  more  scoriaceous  on  the  surface;  and  the  pores  are 
found  to  diminish  in  number  and  size  as  those  portions  of  the  original 
molten  magma  that  were  under  greater  and  greater  pressure  are  reached. 
A  concomitant  of  the  formation  of  the  gas  pores  is  the  relatively  rapid 
cooling  of  the  magma,  producing  rocks  of  a  glassy  nature  or  of  very  fine 
grain.  Both  of  these  conditions  commonly  confront  us  in  A^olcanic  rocks 
or,  in  other  words,  in  rocks  that  have  overflowed  upon  the  surface.  It  is 
true  that  amygdules  have  been  observed  in  rocks  which  occur  clearly  as 
sills  and  dikes,  and  which  therefore  never  actually  reached  the  surface  in  a 
molten  condition.  These  cases  are,  however,  relatively  rare,  and  one  can 
readily  see  that  the  enormous  reduction  of  pressure  occasioned  by  the  intru- 
sion of  the  sills  into  their  present  places  from  much  lower  positions  would 
readily  permit  the  expansion  of  the  included  gas.  Moreover,  in  such  cases 
amygdules  are  far  from  numerous,  showing  that  the  pressure  was  dimin- 
ished to  such  extent  that  relatively  few  pores  were  foiTued.  In  the  case  of 
the  amygdaloidal  greenstones  in  the  Vermilion  district  we  observe  the 
following  conditions:  First,  almost  universally  when  amygdules  are  present 
they  occur  in  great  quantity  and  are  very  commonly  of  large  size ;  second, 
the  amygdaloidal  structure  accompanies  a  fine-grained  condition  of  the  rock. , 


ELY  GREENSTONE.  141 

•The  combination  of  these  two  characters  and  their  g-eneral  distribntion 
among-  the  greenstones  seem  to  indicate  that  these  greenstones,  in  great 
measnre  at  least,  were  poured  forth  at  the  surface. 

THE    SPHERTXLITIC    STRUCTURE. 

The  Ely  g-reenstones  are  very  frequently  marked  by  small,  rounded, 
raised  areas,  which  differ  in  color  from  the  matrix,  being  either  lig-liter 
or  darker.  They  range  in  size  from  that  of  a  pin's  head  up  to  3  inches 
in  diameter.  No  structure  is  visible  on  the  very  small  areas.  They 
merely  stand  out  on  the  weathered  surface  of  the  rock  as  so  many  small 
nodules,  their  relief  being  the  result  of  differential  weathering.  On  the 
larger  bodies,  however,  a  distinctly  radial  arrangement  can  be  seen,  and 
this  is  especially  well  shown  on  weathered  siirfaces.  The  essential 
characteristic  of  spherulites  is  tliat  they  are  formed  of  radiating  or 
diverging  groups  of  crystals  which  commenced  to  crystallize  from  one 
point  or  a  center.  These  are  the  characteristics  of  the  objects  mentioned. 
They  are  similar  in  general  characters  to  the  spherulites  of  the  acid  lavas, 
but  differ  from  them  in  mineral ogic,  and  hence,  of  course,  in  chemical, 
composition.  One  can  see  witli  the  naked  eye  that  chlorite  in  radiating 
fibers  is  the  chief  constituent  of  some  of  the  spherulites.  Most  of  them 
are  formed  of  a  grayish  material  whose  character  can  not  be  recognized 
macroscopically.  The  characters  of  these  spherulites  as  seen  under  the 
microscope  will  be  described  below  (see  p.  152).  From  the  pubhshed 
descriptions  of  various  occurrences  of  spherulites  it  appears  that  they 
are  generally  found  in  rock  masses  that  are  believed  to  be  flows,  or,  more 
rarely,  upon  the  selvage  of  small  dikes.  So  true  is  this  that  spherulites, 
like  porous  and  slaggy  sti'uctures,  have  come  to  be  considered  as  fair 
evidence  of  the  original  extrusive  character  of  rocks  in  which  they 
occur.  This  spherulitic  structure  is  not  found,  however,  accompanying 
the  amygdaloidal  rocks  in  the  Vermilion  district.  On  the  contrary,  the 
conditions  for  the  formation  of  gas  pores  in  large  quantity  seem  to  have 
precluded  the  formation  of  spherulites,  although  some  few  vesicles  may 
occur  with  the  spherulites.  On  one  good  exposure  a  traverse  showed 
the  fine-grained  amj^gdaloidal  rock  grading  downward  into  a  rock  growing 
gradually  coarser  and  coarser,  in  which  the  amygdules  disap^^ear,  and  when 
they   had    completely  disappeared    the    spherulites   were    found    to    have 


142  THE  VERMILION  IRON-BEARING  DISTRICT. 

developed  in  large  quantity.  These  pre-Cambrian  basic  lavas  exhibit  con- 
ditions almost  exactly  the  same  as  those  observed  by  Iddings  in  the  acid 
lavas  of  much  more  recent  times  in  the  Yellowstone  Park  and  described 
by  him  as  follows:" 

In  recapitulation,  then,  this  rhyolitic  lava  is  a  flow  about  100  feet  thick,  except 
where  it  has  piled  up  in  a  small  valley.  It  is  glassy,  except  the  lithoidal  portion 
in  the  valle}',  and  is  fi'ee  from  phenocrysts.  The  obsidian  is  dense  in  a  lower  part 
of  the  edge  and  carries  numerous  spherulites.  Large  vesicles  occur  in  the  upper 
portion,  and  toward  the  surface  of  the  flow  the  spherulites  disappear  and  the  glass 
become.'?  filled  with  gas  cavities  and  passes  up  into  pumice.  *  *  *  These 
characteristics  repeat  themselves  in  the  rhyolite  in  various  parts  of  the  park. 

Up  to  the  present  time  there  have  been,  to  m)^  knowledge,  only  two 
occurrences  of  basic  spherulites  or  spherulitic  rocks  (variolites)  described 
from  North  America.  The  one  was  by  Ransome  from  California ''  and  the 
other  by  myself  from  the  northern  peninsula  of  Michigan."  In  the  case 
of  the  Ely  greenstone  the  spherulitic  structure  is,  as  has  already  been 
intimated,  one  of  the  most  common,  most  characteristic,  and  most  striking 
features  of  the  rock.  The  spherulitic  greenstones  are  distributed  in  dis- 
continuous exposures  over  a  great  number  of  square  miles.  For  instance, 
they  are  very  common  and  beautifully  developed  on  the  bare  hills  north 
of  Long  Lake  in  sees.  10,  11,  14,  15,  16,  21,  and  22,  T.  63  N.,  R.  12  W. 
They  are  especially  common  just  southeast  of  Jasper  Lake  in  sees.  1  and 
12,  T.  63  N.,  R.  10  W.,  and  sec.  6,  T.  63  N.,  R.  9  W.  They  are  also 
very  common  west  of  North  Twin  Lake  in  sees.  10,  11,  14,  and  15, 
T.  63  N.,  R.  10  W.  Well-developed  spherulites  occur  also  about  a  mile 
north  of  North  Twin  Lake  at  the  northeast  corner  of  sec.  12,  T.  63  N., 
R.  10  W.  In  many  of  the  places  mentioned,  especially  in  sec.  6,  T.  63  N., 
R.  9  W.,  the  exposures  are  almost  solid  masses  of  spherulites,  which  show 
very  beautifully  on  the  weathered  surface  their  radial  structure.  Figures 
A  and  B  of  PL  III  are  illustrations  made  from  a  polished  and  a  weathered 
specimen,  respectively,  of  these  spherulitic  rocks,  and  give  a  correct  idea  of 
their  appearance. 

"  Geology  of  the  Yellowstone  National  Park,  Descriptive  Geology,  Petrograpliy,  ami  Paleoutology, 
by  Hague,  Iddings,  Weed,  Waleott,  Girty,  Stanton,  and  Knowlton:  3Ion.  U.  S.  Geo).  Survey  Vol. 
XXXII,  Part  II,  1S99,  p.  365. 

''  The  eruptive  rocks  of  Point  Bonita,  by  F.  Leslie  Eansome:  Bull.  Tniv.  of  Cal.,  Vol.  I,  1S03,  p.  99. 

''The  Crystal  Falls  iron-l^earing  district  of  Michigan:  Jlon.  U.  S.  Geol.  Survey  Vol.  XXXVI, 
1899,  p.  108. 


U.   S.   GEOLOGICAL  SURVEY 


MONOGRAPH  XLV       PL.    Ill 


SPHERULITIC   TEXTURE    IN   THE   GREENSTONES. 

A.  Fragment  taken  from  the  periphery  of  a  spherulitic  ellipsoid.     The  outside  of  the  ellipsoid  is  made  of  the  finest-grained  material,  with  an  occasional 

soherullte.     Near  the  center  the  spherulites  are  more  numerous  and  larger. 

B.  This  illustrates  the  spherulitic  character  of  many  of  the  greenstones,  especially  those  which  show  the  ellipsoidal  parting. 


ELY  GREENSTONE.  143 

When  the  clearly  recognizable  characters  of  the  spherulites,  as  shown 
in  the  above  illnstrations,  and  the  extent  of  the  distribution  of  these 
spherulitic  greenstones  are  taken  into  consideration,  it  is  with  very  great 
surprise  that  one  finds  that  the  only  recognition  which  the  spherulites 
have  received  was  by  N.  H.  Winchell,  who  states,  in  his  report,"  that  the 
surface  of  the  rock  is  mottled  by  small  areas  of  lighter  color  than  the  matrix 
in  which  they  lie,  and  refers  to  them  as  indicating  an  original  amygdaloidal 
or  fragmental  structure.  Spherulites  are  now  known  to  exist  in  the  ancient 
acid  volcanics  over  various  regions  of  the  United  States.  They  have  also 
been  described  from  numerous  localities  where  more  recent  acid  lavas  are 
developed.  Iddings  well  states  the  probable  reason  for  the  more  frequent 
occurrence  of  such  crystallizations  in  acid  than  in  basic  lavas  in  the  following 
words  -.^ 

The  greater  frequencj'  of  lamination  and  localized  erystallization  in  acid  lavas 
as  compared  with  basic  ones  is  a  consequence  of  the  generally  greater  viscosity  of  acid 
lavas  at  the  time  of  their  eruption.  The  basic  rocks  have  a  considerably  lower  melting 
point  and  are  much  more  liquid  up  to  the  temperature  of  solidification.  Hence, 
diffusion  would  take  place  more  rapidh'  and  the  magna  would  be  more  homogeneous, 
other  things  being  equal. 

The  spherulitic  metabasalts  or  greenstones  are  extraordinarily  abun- 
dant in  the  Vermilion  district.  They  have  a  very  great  development  in  the 
adjacent  portions  of  Ontario  underlain  by  greenstones.  The  spherulitic 
structure  occurs  in  similar  Huronian  rocks  in  the  Crystal  Falls  district 
of  Michigan,  and  is  likewise  developed  in  the  rocks  of  the  Menominee 
district  of  Michigan.  Mr.  C  K.  Leith  reports  the  occurrence  of  similar 
rocks  in  the  Mesabi  district  of  Minnesota.  Considering  the  extraordinarily 
widespread  development  of  this  structure  in  the  areas  mentioned,  one  is 
led  to  wonder  at  the  fact  that  it  is  not  present  in  similar  rocks  which  have 
a  widespread  occurrence  in  the  Penokee-Grogebic  district  of  Michigan  and 
Wisconsin,  and  in  the  Marquette  district  of  Michigan.  Furthermore,  it 
seems  surprising  that  this  structure  should  not  exist  in  the  somewhat 
more  recent  metabasalts  of  the  Keweenawan  of  the  Lake  Superior  region, 
and  in  the  still  more  recent  basalts  of  the  Triassic  of  the  Atlantic  coast. 
It  seems  highly  probable  that  this  spherulitic  structure  must  exist,  at  least 

«Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Rent,  Vol.  IV  1S99,  p.  253. 
6  Geology  of  the  Yellowstone  National  Park;  De'crip  ive  Geology,  Pttrology,  anil  Paleontology, 
by  Hague,  Iddings,  and  othen-. :  Mon.  U.  S.  Gi;ol.  Survey  Vol.  XXXIl,  Part  II,  1899,  p.  4:'.5. 


144  THE  VERMILION  IRON-BEARING  DISTRICT. 

iu  a  limited  development,  in  the  rocks  of  the  areas  mentioned,  and  that 
its  occurrence  has  simply  been  overlooked,  or  else  that  the  spherulites 
were  not  recognized  as  such,  but  were  called  amygdules,  as  they  were  in 
the  notes  of  some  of  the  observers  in  the  Vermilion  district  before  attention 
was  called  to  the  true  character  of  these  bodies. 

THE    ELLIPSOIDAL"    STRUCTUEE. 

The  ellipsoidal  structure  iu  pre-Cambrian  greenstones  (metabasalts) 
was  described  by  the  author  in  1899,*  and  the  attempt  was  made  to  account 
'for  its  occurrence.  It  was  concluded  that  the  greenstones  correspond  in 
general  characters  and  in  mode  of  origin  to  aa  lava,  and  that  the  ellipsoids 
were  due  to  the  breaking  up  of  this  relatively  viscous  lava.  It  was 
concluded  -that  the  shajie  of  the  ellipsoids  was  determined  to  a  great  extent 
by  the  rolling  over  and  over  of  these  units  and  the  pressure  under  which 
they  existed  at  this  time  as  well  as  their  cooling  and  consequent  contraction, 
witli  possibly,  as  an  additional  and  less  nnportant  factor,  the  pressure  to 
which  they  were  subjected"  subsequent  to  their  complete  cooling.  In  a 
recent  article  Gregory  describes  the  ellipsoidal  structure  in  Maine  andesites, 
and  writes  of  the  brecciated  rock  (glassy  and  stony)  which  fills  the 
interstices  between  the  ellipsoids."  This  is  confirmatory  of  my  statement 
that  the  matrix  between  the  ellipsoids  in  the  Lake  Superior  region  was 
originally  a  breccia,  in  part  at  least,  which  is  now,  however,  as  the  result 
of  pressure,  almost  always  distinctly  schistose.  The  conclusion  as  to  this 
orio-inal  brecciated  character  of  the  matrix  was  reached  chiefly  as  the 
result  of  microscopic  study  of  thin  sections  of  the  schistose  matrix.''  A 
clastic  matrix  has  been  observed  filling  the  interstices  of  rocks  with  similar 
elhpsoidal  structure  and  is  found  described  in  the  work  of  Geikie  on  the 

«  The  advantage  ot  using  the  term  ' '  ellipsoidal, ' '  applied  to  designate  the  peculiar  parting  found 
in  some  of  the  basic  igneous  rocks,  was  emphasized  in  the  description  of  the  Crystal  Falls  iron-liearing 
district  of  Michigan :  Mon.  U.  S.  Geol.  Survey  Vol.  XXX VI,  1899,  pp.  112-124.  The  writer  has  observed, 
in  conversation  with  various  geologists,  that  in  practically  every  case  in  which  the  term  "spheroidal" 
was  applied  to  this  parting,  the  person  using  it  had  the  idea  that  it  corresponded  very  closely  to  the 
secondary  spheroidal  parting  which  is  so  well  known  in  all  igneous  rocks.  Very  naturally  confusion 
is  thus  caused  in  the  minds  of  the  geologists  by  the  use  of  the  term  spheroidal  to  designate  the  original 
parting  in  the  rocks  on  the  one  hand,  and  the  structure  produced  as  the  result  of  weatliering  processes 
on  the  other.  It  seems  to  me,  therefore,  more  than  ever  necessary  to  confine  the  term  ellipsoidal  to 
this  original  parting  and  the  term  spheroidal  to  the  structure  produced  by  weathering. 

f'Mon.  U.  S.  Geol.  Survey  Vol.  XXXVI,  1899,  pp.  112-124. 

c  Am.  Jour.  Sci.,  4th  series.  Vol.  VIII,  1899,  p.  367. 

i'  Loc.  cit. 


ELY  GREENSTONE.  145 

ancient  vqlcanics  of  Great  Britain."  In  some  of  the  cases  mentioned  by 
him,  however,  this  clastic  matrix  is  clearly  of  sedimentary  origin,  as  lines 
of  sedimentation,  with  separation  into  the  finer  and  coarser  grained  material, 
are  clearly  recognized.  In  none  of  the  ntimerons  cases  studied  in  the  Lake 
Superior  region  is  there  any  indication  that  this  clastic  material  is  of 
sedimentary  origin,  hence  it  has  been  concluded  that  it  was  due  to 
brecciation.  The  reservation  must  be  made,  however,  that  some  of  it  inay 
well  be  a  tuff  deposit  in  which,  as  the  result  of  the  small  amount  that  can 
be  studied,  no  differentiation  in  grain,  etc.,  is  shown. 

The  mode  of  formation  of  these  ellipsoids,  as  suggested,  and  the 
presence  in  them  of  great  quantities  of  amygdules,  seem  to  point 
conclusively  to  the  fact  that  the  lava  in  which  they  occur  is  a  surface 
flow,  although  the  flows  may  have  been  of  submarine  formation. 

Ellipsoidal  basalts  identical  in  every  way  with  those  from  Michigan, 
whose  characters  have  been  already  described,  occur  in  the  Vermilion 
district  of  Minnesota,  and  are  very  widespread.  An  occurrence  at  one 
locality  was  described  several  years  ago  by  WinchelL'  More  recently,  in 
the  last  volume  of  the  Minnesota  report,"  a  number  of  other  localities  in 
the  Vermilion  district  have  been  enumerated,  in  which  rocks  having  this 
structui'e  occur.  The  greenstones  possessing  this  structure  have,  however, 
a  much  wider  distribution  in  the  Vermilion  district  than  would  be  inferred 
from  the  description  given  by  the  State  geologist.  Corresponding  to  their 
wide  distribution  in  the  Vermilion  district  proper  they  have  also  been  found 
by  reconnaissance  to  cover  large  areas  in  the  adjacent  portion  of  Canada 
forming  the  continuation  of  this  district.  This  distribution  is  practically 
the  same  as  that  of  the  Ely  greenstones,  for  this  structure  can  be  seen  in 
more  or  less  perfect  development  on  nearly  all  of  the  large  exposures  of 
that  rock.  The  accompanying  illustration  (PI.  IV,  A)  shows  nothing 
essentially  different  from  the  sketches  reproduced  in  Monograph  XXXVI, 
but  is  taken  from  a  photograph  and  is,  therefore,  of  much  greater  value. 
The  photograph  represents  an  exposure  about  50  paces  south  of  the  county 
road  1  mile  east  of  Soudan,  just  northeast  of  Jasper  Peak. 

The  various  observations  recorded  in  Monograph  XXXVI  concerning 

a  Ancient  Volcanics  of  Great  Britain,  by  Sir  Archibald  Geikie,  Vol.  I,  1898,  pp.  18-1  and  193. 
''The  Kawishiwin  agglomerate  at  Ely,  r\linnesota:  Am.  Geologist,  Vol.  IX,  1892,  pp.  359-368. 
cGeol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  pp.  255-267,  274,  276. 

MON  XLV — 03 10 


146  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  peripheral  and  concentric  an-ang-ement  of  the  amj^gdules  in  the  ellipsoids 
were  confirmed  by  repeated  observations  ou  similar  occurrences  in  the 
Vermilion  district.  At  one  place  on  the  hill  just  west  of  Ely  and  not  very 
far  from  the  last  house  in  the  town,  it  was  noticed  that  the  arnvgdules  were 
concentric  ou  one  side  of  the  ellipsoids,  although  a  few  were  scattered 
through  the  ellipsoids.  In  this  case  the  exposure  showed  a  transition  from 
amygdaloidal  ellipsoidal  rock  to  amygdaloidal  nonellipsoidal  basalt,  both  of 
essentially  the  same  grain.  The  exposure  is  about  20  paces  in  width  north 
and  south.  It  looks  very  much  as  though  the  ellipsoidal  portion  of  this 
ancient  lava  represents  the  surface  of  the  flow,  which  was  more  viscous 
than  the  inner  portion  and  consequently  more  readily  broken.  This 
being  true,  such  a  relationship  readily  explains  the  occm-rence  of  a  transi- 
tion from  ellipsoidal  to  nonellipsoidal  foi'ms  of  the  same  rock,  as  discussed 
in  detail  in  Monograph  XXXVI,  to  which  reference  has  repeatedly  Ijeeu 
made.  Such  gradations  in  these  ancient  lavas  are  shown  by  a  number  of 
observations  taken  at  different  places.  One  passes  in  the  field  from  a  fine- 
grained amygdaloidal  ellipsoidal  basalt  to  an  ellipsoidal  basalt  in  which,  by 
gradual  transition,  the  grain  has  become  considerably  coarser;  it  is  then  an 
ellipsoidal  anamesite,  if  we  use  the  terms  basalt,  anamesite,  and  dolerite  to 
express  the  degrees  of  coarseness  of  crystallization;  it  then  grades  into  a 
coarse-grained  ellipsoidal  dolerite,  which  in  its  turn  grades  into  an  even 
coarser  dolerite  without  marked  ellipsoidal  parting.  Continuing,  this  same 
sequence  is  g'one  over  in  reverse  order,  from  the  coarse,  massive  dolerite  to 
the  fine-grained  ellipsoidal  basalt.  The  best  place  to  get  this  complete 
sequence  is  on  the  bare  hills  south  of  Moose  Lake,  along  the  section  line 
between  sees.  32  and  33,  T.  64  N.,  R.  9  W.  Another  place  in  which  the 
transition  from  the  fine  amygdaloidal  ellipsoidal  basalt  to  the  massive 
dolerite  can  be  excellently  seen  is  north  of  Long  Lake.  It  is  about  500 
paces,  or  one-fourth  mile,  north  of  the  southeast  corner  of  sec.  9,  T.  63  N., 
R.  12  W. 

Observations  show  that  the  amygdules  are  not  the  only  features  which 
are  common  in  the  ellipsoids,  as  the  ellipsoids  are  also  commonly  spherulitic." 
In  fact,  the  spherulites  have  been  observed  only  on  exposures  which  show 
a  more  or  less  perfect  ellipsoidal  parting-.  The  best  spherulites  have  been 
seen  Avliere  the  ellipsoids  are  typically  developed  and  show  the  following 

"Cole  anil  Gregory,  Quarterly  Jour.  Geol.  .Soc,  Vol.  XLVI,  ISVlO,  p.  311. 


U.   S.   GEOLOGICAL  SURVEY 


MONOGRAPH  XLV      PL.    IV 


A.     ELLIPSOIDAL    PARTING    IN    GREENSTONE. 


J?.      ELLIPSOIDALLY    PARTED    GREENSTONE,    SHOWING    SPHERULITIC    DEVELOPMENT. 


ELY  GREENSTONE.  147 

relations  to  these  ellipsoids.  The  smallest  spherulites  occupy  the  extreme 
outside  of  the  ellipsoids.  From  the  outside  they  increase  in  size  toward 
the  center,  where,  if  the  rock  is  there  spherulitic  at  all,  the  largest 
spherulites  are  developed.  Sometimes  the  development  of  the  magma 
into  a  spherulitic  rock  did  not  reach  entirely  to  the  center,  which  is  then 
developed  as  a  massive  dolerite  of  normal  character.  It  is  very  noticeable 
that  while  the  spherulites  occur  in  the  very  fine-grained  lavas  they  are 
apparently  equally  common  in  some  of  the  more  coarsely  crystalized  forms 
when  these  phases  are  ellipsoidal.  The  spherulitic  and  amygdaloidal 
structures  sometimes  occur  together,  but  most  commonly  they  are  not 
developed  in  the  same  rock.  Apparently  the  presence  of  one  does  not 
altogether  preclude  the  existence  of  the  other,  although  it  amounts  very 
nearly  to  this.  Thus,  in  passing  over  the  jjarticular  section  through  the 
ellipsoidal  amygdaloidal  lavas,  which  was  noted  above  as  occurring  north 
of  Long  Lake,  we  find  that  the  amygdaloidal  ellipsoidal  fine-grained  lava 
is  first  nonspherulitic.  Soon,  however,  the  spherulites  begin  to  appear  and 
gradually  increase  in  importance,  the  amygdules  decreasing  correspondingly 
in  quantity  and  the  grain  of  the  matrix  between  the  spherulites  increasing 
also  in  size.  The  spherulites,  however,  are  wanting  in  the  coarsest  non- 
ellipsoidal  phases  of  the  lava.  The  arrangement  of  the  spherulites  in 
concentric  circles  made  up  of  spherulites  of  larger  and  larger  size  as  the 
center  is  approached  shows,  as  does  the  concentric  arrangement  of 
amygdules  in  the  ellipsoids,  that  each  ellipsoid  must  be  reckoned  with  as 
a  unit.  Observations  show  that  small  spherulites  occur  on  the  outside  of 
a  spherulitic  mass,  where  crystallization  continues  for  only  a  short  time, 
while  the  larger  spherulites,  requiring  proportionally  a  longer  period  for 
their  formation,  occur  deeper  down  in  the  rock. 

Magnificent  exposures  of  these  spherulitic  ellipsoidal  greenstones 
occur  on  the  hills  north  of  Long-  Lake  for  the  greater  portion  of  the  distance 
between  this  lake  and  Bass  Lake,  in  sees.  9  and  10,  T.  63  N.,  R.  12  W. 
One  of  the  finest  exposures  seen  was  that  occurring-  on  the  high  hill  about 
500  paces  south  of  the  meander  corner  on  the  shore  of  Bass  Lake.  The 
illustration  presented  in  B  of  PI.  IV  is  made  from  a  photograph  of  this 
exposure  and  shows  the  spherulitic  character  of  the  ellipsoids;  the  eight 
spots  in  the  photograph  are  the  spherulites.  The  somewhat  schistose, 
brecciated  matrix  between  the  ellijDsoids  can  also  be  seen  in  the  illustration. 


148  THE  VERMILION  IRON-BEARING  DISTRICT. 

Other  good  exjDOsures  of  splierulitic,  ellipsoidal  greenstones,  in  wliicli  four 
or  five  rows  of  spherulites  can  be  seen,  occur  in  the  northeast  corner  of 
sec.  7,  T.  63  N.,  R.  11  W.  Perhaps  even  better  ones  can  be  seen  south  and 
southeast  of  Jasper  Lake,  especially  in  sec.  6,  T.  63  N.,  R.  9  W.  Here 
the  hills  are  nearly  bare  and  exposures  extend  almost  continuously  from 
the  south  shore  of  Jasper  Lake  eastward  along  the  section  line  to  the  south 
quarter  post  of  sec.  6,  T.  63  N.,  R.  1 1  W. 

These  ellipsoidal  or  aa  greenstones  have  been  subjected  to  orogenic 
movement,  and  when  in  the  zone  of  fracture"  they  have  been  jointed,  and 
in  places  brecciation  has  also  taken  place.  A  difiFerent  result  followed 
when  the  rocks  were  more  deeply  buried  and  subjected  to  great  pressure, 
which  produced  interesting  structures  that  are  in  some  places  now 
exposed  at  the  surface.  The  ellipsoids  then,  instead  of  being  fractured, 
were  mashed  into  disks,  just  as  one  could  mash  a  Imnp  of  stiff  dough  into 
a  disk-shaped  body.  The  cross  section  of  such  a  mashed  ellipsoidal 
greenstone  shows  that  the  ellipsoids  have  enormously  elongated  axes, 
approximately  parallel  to  the  direction  of  minimum  pressure,  and  a 
proportionally  short  one  in  the  direction  of  greatest  pressure.  Various 
stages  of  this  deformation  have  been  observed.  In  extreme  cases  these 
rocks  have  a  banded  appearance,  the  material  of  the  ellipsoids  forming 
bands  of  dense  material  alternating  with  other  bands  which  consist  of  what 
was  the  matrix  between  the  ellipsoids.  This  rapid  alternation  of  bands  of 
differing  material — the  bands  derived  from  the  matrix  may  be  an  inch  and 
a  half  in  thickness,  the  bands  from  the  ellipsoids  being  usually  a  little 
thicker — very  closely  simulates  true  bedding,  and  miglit  very  readily  be 
construed  as  such  on  a  hasty  examination.  Especially  might  it  be  so  taken 
in  cases  where,  as  freqifently  happens,  the  exposures  are  onlj*  a  few  square 
feet  or  at  most  a  few  square  yards  in  area.  If  one  had  large  areas  of  these 
massive  ellipsoids  to  study,  however,  the  bands,  if  examined  in  detail,  would 
be  found  to  be  relatively  short  and  to  be  made  up  of  enormously  elongated 
lenses.  Such  large  exposures  are,  however,  rare.  One  of  the  best  exposures 
of  rock  of  this  soi-t  is  on  the  high  ridge  south  of  the  exjjloring  camp  on 
the  south  side  of  Moose  Lake.  The  rock  here  is  the  typical  splierulitic 
ellipsoidal  greenstone,  and   shows  very  nearly  clean  exposures  over  an 

"Principles of  pre-Cambrian geology,  by  C.  R.  Van  Hitie:  Si.xteenth  Ann.  Kept. U. S. Geol.  Survey, 
Pt.  I,  1896,  p.  696. 


ELY  GREEN;<TONE.  149 

area  several  hundred  paces  long.  In  the  center  of  this  large  exposure  the 
ellipsoids  are  relatively  little  mashed,  and  show  the  aplierulitic  structure 
within  them  as  well  as  the  matrix  between  them.  This  was  evidently  a 
part  of  the  rock  mass  that  acted  as  a  buttress,  and  was  not  affected  by  the 
mashing,  as  was  the  rock  on  each  side  of  it.  On  the  sides  of  the  ridg-e  the 
ellipsoids  are  much  flattened.  The  rock  in  places  passes  into  a  greenstone- 
schist  in  which  the  ellipsoidal  structure  is  totally  obliterated.  Between 
these  green  schists  on  the  one  hand  and  the  typical  ellipsoidal  greenstone 
on  the  other  there  are  various  gradations.  The  intermediate  phases  show 
a  certain  coarse  banding  which,  by  a  careless  observer,  might  be  mistaken 
for  lines  of  sedimentation.  This  banding  is  produced  in  the  way  indicated 
above,  the  bands  being  of  two  kinds,  one  kind  produced  from  the  matrix 
and  the  other  produced  from  the  original  ellipsoids. 

Still  another  excellent  example  of  this  pseudobedded  structure  may 
be  seen  in  the  Archean  just  north  of  the  railroad  near  the  east  end  of  the 
place  where  the  Duluth,  Port  Arthur  and  Western  Railroad  first  reaches 
the  shore  of  Gunflint  Lake  from  the  east.  This  is  north  of  the  railroad 
track  and  distant  from  it  from  75  to  150  paces.  To  the  north  the  green- 
stone is  fairly  massive,  and  in  places  is  distinctly  ellipsoidal.  Toward  the 
south,  nearer  the  overlying  sedimentaries  and  consequently  nearest  the 
plane  along  which  movement  must  have  taken  place  during  the  folding  of 
the  rocks,  it  becomes  decidedly  schistose.  The  ellijasoidal  masses  are 
flattened  to  such  an  extent  as  to  give  a  rough  banding  to  the  rock. 

This  description  of  the  ellipsoidal  structure  in  these  greenstones  would 
not  be  complete  if  attention  were  not  called  to  the  frequency  of  its  occur- 
rence in  the  various  districts  of  the  Lake  Superior  region.  Thus,  for 
example,  it  has  been  described  from  the  Marquette  and  the  Crystal  Falls 
districts  of  Michigan,  and  one  can  state  with  a  fair  degree  of  assurance, 
from  the  occurrence  of  large  quantities  of  greenstones  in  the  Penokee- 
Gogebic  of  Michigan  and  Wisconsin,  that  it  also  occurs  there,  although  it 
has  not  been  described  from  that  district.  It  has  also  been  observed  by  the 
writer  in  a  number  of  places  in  the  Menominee  district  of  Michigan  and  in 
the  Mesabi  district  of  Minnesota.  Lawson  describes  it  in  the  rocks  of 
the  Lake  of  the  Woods  region."     The  same  structure  has  been  described 

"Geology  of  the  Lake  of  the  AVoods  region,  by  A.  C.  Lawson:  GeoL.and  Nat.  Hist.  Survey  of 
Canada,  1885,  pp.  51-53otf. 


150  THE  VERMILION  IRON-BEARING  DISTRICT. 

from  the  Micbipicoten  iron-bearing  district  on  the  east  side  of  Lake 
Superior  b}^  A.  B.  Wilhnott,"  and  Dr.  S.  Weidman,  of  the  Wisconsin  Greolog- 
ical  and  Natural  Histor}'  Survey,  states  that  it  occurs  in  greenstones  of 
supposed  Huronian  age  in  the  vicinit}-  of  Wausau,  Wis.  It  has  been 
observed  in  the  Archean  greenstones  on  Lake  Nipigon  in  Ontai'io,  Canada. 
As  the  result  of  field  studies  of  the  Keweenawan  volcanics  of  the  north 
shore  of  Lake  Superior  in  1900,  the  writer  knows  that  it  occurs  also  in 
them.  Although  so  very  common  throughout  the  Lake  Superior  region  in 
the  rocks  of  pre-Cambrian  age,  it  appears  to  be  relatively  rare  in  the  petro- 
graphically  similar  rocks  of  later  age  found  elsewhere  in  North  America. 
This  structure  has  been  found  to  be  so  common  throughout  the  Lake 
Superior  region  that  it  is  now  considered  characteristic  of  the  j^re-Cambrian 
greenstones  of  the  region.  It  is  not,  however,  confined  to  any  one  of  the 
divisions  of  the  pre-Cambrian  rocks.  The  rocks  in  which  it  occurs  range 
from  the  Archean  of  the  ^^ermilion  district  of  Minnesota  and  the  adjacent 
Canadian  districts  and  the  Marquette  district  of  Michigan,  to  the  Keween- 
awan,. It  occurs  within  the  greatest  surficial  areas  of  the  Archean.  This 
same  structure  has  been  found  by  Ceikie  in  the  lavas  of  Great  Britain.* 
In  a  letter  to  the  writer  Geikie  says:  "This  remarkable  structure  appears  to 
be  far  more  common  in  lavas  of  all  ages  than  I  supposed.  It  is  admirably 
developed  in  our  Arenig  lavas,  and  I  have  lately  found  it  in  those  of  the 
Old  Red  sandstones  and  Carboniferous  system." 

MICROSCOPIC  CHARACTERS. 

The  rocks  composing  the  Ely  greenstone  have  been  divided  according 
to  their  macroscopic  characters  into  the  porphyritic  and  non-porphyritic 
varieties,  the  normal  diabasic  or  ophitic  textured  forms,  the  amygdaloidal, 
spherulitic,  and  ellipsoidal  forms.  Stress  has  been  laid  upon  some  of  the 
principal  macrosco])ic  characters,  and  these  divisions  have  been  made 
merely  for  the  purpose  of  aiding  in  the  study  of  the  rocks  and  not  because 
the  varieties  were  distinguished  liy  important  diftereuces  in  microscopic 
characters,  except  in  a  few  cases.  As  the  reader  would  infer  from  their  age 
and  from  the  use  of  the  name  greenstone  in  connection  with  them,  these 


"The  Michipicoten  Huronian  area,  by  A.  B.  Willniott:  Am.  Geologist,  Vol.  XXVIII,  1901,  p.  U. 
The  uomenclatnre  of  the  Lake  Superior  I'orniation.s,  by  A.  B.  Wiluiott:  Jour.  Ueol.,  Vol.  X,  1902, 
p.  71. 

''Am-ieiit  N'oleauics  of  Great  Britain,  liy  Sir  Archibald  Geikie,  Vol  1,  ISSKS,  pp.  184  and  193. 


ELY  GREENSTONE.  151 

rocks  are  very  much  altered.  The  original  minerals  that  remain  are 
very  few.  The  microscope  discloses  the  following  original  constituents: 
Hornblende,  augite,  feldspar,  quartz,  titaniferous  magnetite,  and  apatite. 
The  original  hornblende  is  the  common  brown  variety.  The  augite  varies 
from  yellow  to  yellowish  green  and  possesses  its  normal  characters.  The 
feldspar  usually  shows  broad  twinning  lamellse,  although  in  some  cases 
it  was  found  in  imperfect  sheaves.  In  one  case  it  was  distinctly  seen 
to  have  been  formed  prior  to  the  titaniferous  magnetite,  as  the  magnetite, 
occurring  in  large  plates,  incloses  lath-shaped  feldspars.  The  feldspar  is 
generally  very  much  decomposed,  so  much  so  that  one  can  not  determine 
its  exact  characters.  It  is  presumed  to  be  a  labradorite.  There  is  very 
little  quartz,  but  some  was  found  occurring  in  micropegmatitic  intergrowth 
with  the  feldspar,  and  is  presumed  to  be  a  primarj^  constituent.  Sometimes 
it  fills  in-egulai-  interstices  between  the  other  minei-als  as  primary  quartz 
representing  the  last  product  of  the  crystallization  of  the  rock.  Magnetite 
and  apatite  show  nothing  uncommon. 

The  secondary  constitiients  are  common  green  hornblende,  actinolite, 
biotite,  chlorite,  sericite,  epidote,  zoisite,  sphene,  rutile,  feldspar,  quartz, 
pyrite,  and  hematite.  The  feldspar  has  iisually  altered  to  a  mass  of  sericite, 
kaolin  (?),  feldspar,  and  quartz.  In  some  cases  it  is  completely  saussuritized. 
There  were  observed  occasional  irregular  but  in  general  rounded  serpeu- 
tinous  areas,  which  are  strongly  suggestive  of  aggregates  of  olivine  indi- 
viduals in  which  the  olivine  possesses  no  definite  crystallographic  outline. 

TEXTURE. 

The  great  majority  of  the  greenstones  are  massive  rocks,  varying  from 
fine  to  coarse  in  grain.  The  textures  they  originally -possessed  have  to  a 
certain  extent  been  obscured  by  the  various  processes  of  alteration  to  which 
they  have  been  subjected.  Both  fine-  and  coarse-grained  greenstones  and 
all  of  the  intermediate  phases  show  locally  porphyritic  texture,  the  pheno- 
crysts  being  usually  of  feldspar,  but  occasionally  of  brown  hornblende. 
In  the  coarse-grained  rocks  the  ophitic  texture  predominates;  in  the 
even-textm-ed,  fine-grained  rocks  the  following  are  commonly  developed: 
Ophitic,  microophitic,  intersertal,  pilotaxitic,  hyalopilitic,  flowage,  and 
spherulitic  textures.  The  ophitic  and  microophitic  textures  are  the  most 
common,  and  the  mineralogic  composition  is  generally  that  so  characteristic 


152  THE  VERMILION  IKON-BEARING  DISTRICT. 

of  the  nietadolerites  (diabase)  and  metabasalts.  The  rocks  possessing  these 
textures  occur  in  very  large  quantity  throughout  the  district.  These 
textures  are  common  in  the  recent  Ijasahs.  The  raineralogic  composition 
of  the  rocks  is  also  the  same  as  would  be  produced  in  recent  basalts  by 
alteration.  Hence,  in  the  absence  of  chemical  analyses,  the  wi-iter  feels 
warranted  in  asserting  that  the  greater  portion  of  these  greenstones  was 
derived  from  the  alteration  of  originally  basaltic  rocks. 

Spherulitic  texture  is  fairlv  common  in  these  altered  basalts,  and  on 
account  of  its  somewhat  greater  interest  deserves  a  little  more  detailed  con- 
sideration than  has  been  given  to  the  others.  The  spherulite  occasionally 
has  at  its  center  a  very  small  crystal  of  plagioclase  surrounded  by  fine 
sheaves  of  feldsjaar,  and  these  spherulites  are  ver}^  similar  to  those  described 
some  years  ago  from  Michigan."  The  feldspars  are  brownish  when  seen 
under  low  power  and  grayish  when  examined  by  high  power,  as  the  result 
of  the  innumerable  minute  crystals  of  epidote,  a  few  hornblende  individuals, 
and  reddish-brown  to  black  spots  of  ferruginous  material.  Other  spherulites 
consist  largely  of  feldspar,  but  between  the  feldspars  occur  needles  of 
actinolite,  which  seem  to  have  been  derived  from  some  original  ferromag- 
nesiaii  mineral  which,  with  the  feldspars,  formed  the  spherulite.  There  were 
found  in  one  case  in  a  much  altered  greenstone,  instead  of  the  usual  feldspar 
spherulites,  radial  masses  of  rich  green  chlorite  with  silky  luster.  The 
mici'oscope  showed  a  few  crystals  of  magnetite  and  some  epidote  in  these 
spherulites  in  addition  to  the  chlorite. 

With  the  above  kinds  of  rocks,  which  are  unquestionably  of  basaltic 
character,  there  are  rocks  that  possess  an  intei'sertal,  pilotaxitic,  and  hyalo- 
pilitic  texture,  in  some  of  which  porphyritic  feldspars,  occurring  in  isolated 
individuals  or  in  groups,  are  very  common.  In  these  rocks  there  seems  to 
be  a  large  proportion  of  brown  hornblende,  sometimes  occurring  as  pheno- 
crysts.  The  general  appearance  of  the  rocks  is  like  that  of  the  andesites. 
It  appears  that,  associated  with  the  basalts  and  playing  a  subordinate  role 
in  this  district,  there  are  rocks  of  intermediate  composition  which  were 
originally  andesites — both  hornblende-  and  pyroxene-andesites — and  that 
we  are  justified  in  stating  that  meta- andesites  form  a  part  of  the  Ely  green- 
stone. 


"The  Crystal  Falls  iron-bearing  district  of  Michigan,  by  J.  Morgan  Clements:  ]\ron.  V.  S.  Geol. 
Survey  Vol.  XXXVI,  1899,  p.  111. 


ELY  GREENSTONE.  153 


SCHISTOSE    GREENSTONES. 


The  various  greenstones  thus  far  described  Jiave  been  very  much 
changed  by  chemical  action,  as  is  shown  by  the  number  of  secoiidary 
minerals  which  now  replace  the  original  ones.  In  many  cases  dynamic 
action  subsequent  to  or  accompanied  by  the  above  changes  has  produced 
schistose  forms  of  these  greenstones.  These  schistose  greenstones  are  not 
nearly  so  common,  however,  in  the  Vermilion  district  as  one  would  be  led 
to  suppose  from  a  perusal  of  the  literature  which  has  been  published  on  this 
district.  In  this  literature  these  rocks  have  frequently  been  spoken  of  as 
greenstone-schists.  This  term  seems  to  the  writer  to  convey  a  wrong  idea 
of  the  character  of  the  rocks  as  a  whole,  though  it  is  fitting  in  certain  cases. 
The  greenstone-schists  or  green  schists  are  in  reality  a  very  subordinate 
phase  of  the  Ely  greenstone,  and  in  the  great  majority  of  cases  are  of  purely 
local  occurrence  and  very  subordinate  extent.  They  have  been  formed 
along  zones  of  excessive  deformation  and  grade  into  massive  granular  rocks. 
For  this  reason  the  term  schistose  greenstone  has  been  23referred  to  indicate 
them.  In  these  schistose  rocks  all  of  the  original  minerals  have  been 
changed  by  metasomatic  action,  and  as  a  result  of  movement  in  the  rocks 
produced  b}^  shearing  stresses,  the  original  textures  have  also  been  almost 
completely  obliterated. 

GENERAL    CHARACTERS. 

These  rocks  are  schistose  in  character  and  appear  in  various  shades  of 
gTcen.  Only  one  macroscopic  structure  has  been  observed  which  would 
lead  to  the  determination  of  the  original  characters  of  the  rocks.  An  imper- 
fect, nearly  obliterated,  amygdaloidal  structure  was  observed  in  one  case. 
A  microscopic  study  of  the  rocks  shows  that  the  constituents  are  small  and 
the  rocks  very  dense  in  texture.  The  various  minerals  to  be  enumerated 
generally  have  their  long  directions  approximately  parallel,  this  arrangement 
producing  a  schistose  structure.  In  some  cases  almost  complete  recrystal- 
lization  seems  to  have  taken  place,  and  in  these  cases  larger  individuals  have 
been  produced.  Occasionally  the  hornblende  and  chlorite  appear  in  large 
porphyritic  individuals  inclosing  other  constituents  of  the  rock.  Constituents 
of  these  rocks  are  biotite,  muscovite,  chloi'ite,  sericite,  calcite,  epidote, 
zoisite,  pp'ite,  and  limonite.  In  some  cases  the  secondarily  produced 
hornblende  has  undergone  a  tertiary  change  and  has  been  chloritized. 


154  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  above  rocks,  when  examined  in  the  laboratorj^,  give  no  cine  to  the 
character  of  the  rocks  from  which  they  were  derived,  if  we  except  the  case 
of  the  specimen  containing  the  amygdviles.  Classified  according  to  their 
mineralogic  composition  we  would  call  them  chlorite-,  amphibole-,  and 
biotite-schists,  and  gneisses.  When  studied  in  the  field,  however,  their 
intimate  relations  with  the  greenstones  and  the  gradations  observed  between 
the  schists  and  the  massive  greenstones  prove  conclusively  that  they  have 
been  derived  from  rocks  similar  to  those  from  which  the  massive  greenstones 
that  now  predominate  throughout  the  district  have  been  derived — in  other 
words,  from  dolerites,  basalts,  and  andesites. 

ORIGIK  OF  ELY  GKEENSTOIs^E. 

There  can  certainly  be  no  reasonable  doubt  in  the  mind  of  the  reader 
as  to  the  original  character  of  the  rocks  described  as  constituting  the  Ely 
greenstone  of  the  Vermilion  district  of  Minnesota.  The  various  textures 
and  structures  that  the  rocks  possess  are  such  as  are  present  onh^  in  igneous 
rocks.  However,  even  though  it  may  be  conceded  that  the  greenstone 
formation  is  of  igneous  origin,  there  remain  still  the  further  queries:  Are 
the  rocks  constituting  it  intrnsi^•e  or  effusive  in  their  character,  or  are 
rocks  of  both  of  these  modes  of  formation  present  in  tlie  complex? 
Furthermore,  if  both  occur,  which  mode  of  origin  is  the  2:)i'edominant  one? 
These  points  may  well  be  discussed  here.  To  the  first  query  the  answer 
must  be  given  that  the  observations  recorded  show  that  both  kinds  of 
rocks — both  effusive  and  intrusive — are  present.  The  answer  to  the  second 
query,  as  to  which  of  these  predominates,  can  be  given  only  with  some 
doubt,  as  it  is  very  difficult  to  make  a  quantitative  estimate  of  the  areal 
distribution  of  rocks  that  are  so  similar  in  character.  From  personal 
observations,  however,  the  writer  has  been  impressed  by  the  Aery  wide 
distribution  of  the  greenstones  that  possess  characters  indicative  of 
effusion.  This  has  led  him  to  place  the  greenstones  with  the  surface  flows; 
but  the  reader  must  be  cautioned  to  include  under  the  term  sin-face  flows 
those  which  maj'  have  been  poured  out  under  water — submarine  flo%\s — 
and  winch  were  thus,  perhaps,  under  relatively  high  pressure,  as  well  as 
those  that  reached  the  suiiace  of  the  land.  In  either  case  the  mode  of 
origin  outlined  for  many  of  these  greenstones  postulates  a  surface  upon  which 
they  could  rest.  Hence,  if  interpreted  in  the  strictest  sense,  they  do  not 
actually  represent  the  original  crust  of  the  earth,  as  Winchell  considers  them 


ELY  GREENSTONE.  155 

to  do."  Nor,  showing  volcanic  characters  as  they  do,  can  they  be  considered 
as  a  part  of  the  earth's  crust  produced  by  downward  crystaUization.  They 
must  be,  indeed,  somewhat  younger  than  the  original  crust.  Nevertheless, 
since  the  greenstones  are  the  basement  upon  which  rests  a  great  series  of 
sediments  that  can  be  correlated  with  sediments  in  other  areas  which  have 
been  regarded  as  of  Lower  Algonkian  age,  we  have  classed  the  igneous 
greenstone  basement  in  the  Vermilion  district  as  of  Archean  ag"e. 

The  intrusives  that  are  considered  to  form  a  part  of  this  complex 
are  those  which  are  of  essentially  the  same  nature  as  the  volcanics,  but 
which  differ  slightly  in  their  mode  of  occurrence.  They  are  those  portions 
of  the  magma  that  penetrated  the  contemporaneous  flows  as  dikes,  and  in 
some  cases,  perhaps,  are  the  material  filling  the  conduits  which  connected 
some  of  the  flows  with  the  magma  mass  from  which  the}"  came.  In  one 
instance  a  frag-ment  of  a  p-reenstone  was  found  included  in  a  somewhat 
difi"erent  greenstone.  The  fragment  was  identified  as  being  similar  to,  and 
presumably  derived  from,  one  of  the  greenstones  of  the  complex.  The 
intrusives  included  in  the  Archean  greenstone  complex  belong  to  the  same 
period  of  formation  as  the  lava  flows  with  which  they  are  associated.  These 
intrusive  rocks  are  of  essentially  the  same  mineralogic  and  chemical  com- 
position as  the  volcanics  themselves. 

It  remains  to  be  stated  that  we  recognize  the  possibility,  and,  indeed, 
the  great  probability,  that  there  have  been  included  in  the  areas  mapped 
as  underlain  by  this  Archean  complex  an  occasional  intrusive  rock 
considerably  younger  than  the  Ely  greenstone  proper.  These  greenstones 
are  cut  by  a  number  of  dikes  of  relatively  recent  age  and  yet  of  essentially 
the  same  character  as  the  greenstones,  except  that  they  are  less  metamor- 
phosed. No  doubt  many  others  were  unrecognized,  and,  indeed,  were 
altogether  unseen  on  account  of  poor  exposures. 

COIS^TACT  METAMORPHISM  OF  ELY  GREEIN^STOKE. 

CONTACT   EFFECT  OF  GRANITE  ON   THE  ELY   GREENSTONE. 

In  the  preceding  pages  general  statements  have  been  made  concerning 
changes  which  the  greenstones  have  undergone  since  they  were  formed 
In  addition  to  those  mentioned,  which  were  essentially  changes  brought 
about  as  the  result  of  ordinary  mountain-making  forces  and  of  percolating 

«The  origin  of  the  Archean  greenstones  of  Minnesota:  Geol.  and  Nat.  Hist.  Survey  of  Minnesota, 
Twenty-third  Ann.  Eept.,  189.5,  pp.  4-24. 


156  THE  VERMILION  IRON-BEARING  DISTRICT. 

waters,  other  changes  of  a  far-reaching-  character  have  taken  place  in  them. 
In  all  of  the  instances  which  will  be  cited  the  chief  agent  of  metamor- 
phism  appears  to  have  been  the  contact  action  of  certain  intrusive  acid 
rocks.  It  is  not  for  a  moment  to  be  supposed,  however,  that  the 
metamorphism  of  the  rocks  should  be  ascribed  solely  to  the  action  of  these 
intrusives;  yet  this  is  the  most  obvious  cause,  and  probably  the  final 
controlling  cause. 

In  the  course  of  the  field  work  on  the  Vermilion  district,  it  was  noticed, 
when  the  exposures  of  greenstone  possessing  the  general  characters  ah-eady 
outlined  for  that  rock  were  studied,  that  a  great  number  of  them  were  cut 
by  dikes  of  acid  rock,  and  that  these  dikes  were  of  varying  size.  It  was 
further  observed  that  near  the  central  portion  of  the  district  these  dikes 
were  relatively  few,  but  that  as  the  southern  and  northern  limits  were 
approached  they  gradually  increased  in  number  until  the  greenstones  were 
in  places  literally  permeated  by  dikes  of  acid  rock.  On  continuing  farther 
from  the  central  part  of  the  district  the  main  body  of  the  granite  was  in 
every  case  finally  reached.  When  this  body  was  reached,  however,  it  was 
found  to  contain  occasional  masses  of  Archean  rocks  of  varying  size,  which 
were  practically  surrounded  by  and  thus  included  in  the  granite.  The 
relations  are  clearly  those  of  intrusion,  a  younger  acid  rock  being'  intrvided 
into  and  including  fragments  from  the  older  Ely  greenstone.  In  brief, 
the  relations  are  the  same  as  those  which  exist  between  the  batholiths  of 
granite  and  the  contiguous  greenstones  of  Rainy  Lake"  and  Lake  of  the 
Woods,  and  which  have  been  so  clearly  described  by  Lawson.  It  was 
further  noted  that  this  intrusion  was  accompanied  by  a  marked  change  in 
the  character  of  the  Archean  complex.  Where  the  granite  dikes  are  few, 
the  characters  of  the  greenstone  formation  remain  essentially  unchanged. 
When  the  dikes  have  become  numerous,  however,  the  greenstones  are 
altered  to  amphibolitic  and  to  a  less  extent  to  micaceous  rocks,  usually  of 
somewhat  darker  color  than  the  normal  greenstones.  The  main  macro- 
scopic characters  are  ])ractically  unchanged.  Thus,  for  example,  in  these 
amphibolitic  rocks  one  can  still  recognize  the  characteristic  ellipsoidal  and 
amygdaloidal  structure  of  the  greenstones.  A  splendid  exposure  of  these 
hornblendic  rocks  can  be  seen  in  the  southeast  quarter  of  sec.  3,  and 
the  northeast  quarter  of  sec.  10,  T.  61  N.,  R.  14  W.     These  rocks,  while 

"Report  on  the  geology  of  the  Rainy  Lake  region:  Geol.  Nat.  Hist.  Survey  Canada,  1S89,  F. 


ELY  GREENSTONE.  157 

possessing  on  the  whole  a  massive  structure,  nevertheless  have,  as  it  were,  an 
incipient  fissility,  or  cleavage,  which  has  been  produced  by  the  process  of 
recrystallization  thi'ough  which  they  have  gone,  as  the  result  of  which  there 
has  been  a  production  of  amphibole  needles  and  chlorite  flakes,  and  also  a 
general  tendency  toward  a  parallel  arrangement  of  the  needles  and  flakes. 
The  secondary  feldspar  has  also  been  affected  in  its  crystallization,  and 
aids  in  emphasizing  the  parallel  structure.  This  parallelism  has  developed 
a  fissility  which  is  not  sufficiently  marked  to  warrant  their  designation  as 
schists.     They  merely  split  more  readily  in  one  direction  than  in  another. 

When  the  contact  between  the  iiiain  granite  masses  and  the  Archean 
greenstone  is  approached,  the  Archean  rocks  are  usually  found  to  have  lost 
all  of  their  characteristic  features  and  to  have  been  recrystallized  into 
amphibolitic  schists  and  gneisses  which  very  rarely  retain  any  recognizable 
greenstone  character.  The  gradation  is,  however,  so  gradual  and  the  steps 
can  be  followed  so  clearly  in  the  field  that  after  a  field  inspection  no  doubt 
as  to  the  con-ectness  of  the  above  conclusions  can  remain  in  the  mind  of 
any  close  and  impartial  observer. 

MINERALOGIC  COMPOSITION   OF  THE  METAMORPHOSED   ROCKS. 

These  amphibole-schists  and  mica-schists,  derived  from  the  green- 
stones, consist  of  the  following  constituents  in  varying  proportions:  Com- 
mon green  hornblende,  actinolite,  biotite,  muscovite,  chlorite,  epidote, 
calcite,  sphene,  quartz,  feldspar,  pyrite,  and  magnetite.  The  mica  is  pres- 
ent in  very  small  quantity  and  is  always  associated  with  amphibole.  It  is 
only  occasionally  that  the  mica  occurs  in  such  quantity  that  the  rock  can 
be  referred  to  as  a  mica  schist.  Banding  is  very  commonly  present, 
as  the  concentration  of  some  of  the  darker  minerals  was  greater  in  certain 
portions  than  in  the  areas  immediately  adjacent. 

The  origin  of  these  metamorphosed  greenstones,  now  schistose  amphi- 
bolitic rocks,  is  such  as  would  be  expected  from  their  distribution  and  from 
their  relationship  to  the  granites.  They  always  lie  between  the  normal 
greenstones  and  the  granites,  occupying  a  belt  of  varying  width,  which 
it  is  impossible  in  the  field  to  delimit  sharply.  This  zone  of  schists  has 
therefore  been  only  approximately  indicated  on  the  maps. 

The  presence  of  these  amphibolitic  schists  adjacent  to  the  granite  has 
been  noticed  by  nearly  every  observer  who  has  been  in  this  district.     On 


158 


THE  VERMILION  IRON-BEARING  DISTRICT. 


a  manuscript  map  by  Irving-  this  belt  of  schists  is  duThued.  Special 
attention  has  been  called  to  them  by  both  A.  and  X.  H.  Winchell  in 
tlieir  published  reports.  The  earliest  explanation  oflPered  was  that  of 
A.  Wincliell,"  who  studied  the  good  exposures  of  these  schists  upon  Burnt- 
side  Lake  and  there  found  them,  as  has  been  described,  permeated  by  the 
granite.  Figs.  1  and  2,  from  his  report,  illusti-ate  the  occurrence.  His 
conclusion  was  that  they  were  derived  from  gray wackes  by  metaraoi-phism. ' 


Fig.  1. — Reijroduction  of  steteh  by  A.  Winchell,  shoivlng  the  intricate  relationship  between  the  granite  of  Burntside  Lake 

and  the  umphibole-schists. 

N.  H.  Winchell "  refers  to  this  belt  of  schists,  and  concludes  that  they 
have  been  "produced  bv  the  granitic  intrusions  or  by  the  force  which 
accompanied  tliem,"  and  that  when  acid  clastic  rocks  were  affected  the 
mica-schists  were  pr.oduced,  and  when  the  basic  greenstone  was  involved 
the  amphibole-schists  ^veve  produced.  T(i  these  rocks  Winchell  applied 
the  name  Coutchiching,  using-  it  in  tlie  sense  proposed  by  Lawson.''     In  his 

"Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Fifteenth  Ann.  Eept.,  1887,  pp.  40-41. 

''Geol.  and  Nat.  Hi.st.  Survey  of  iMinnesota,  Fifteenth  Ann.  Kept.,  18S7,  pp.  172-178.  Ueol.  and 
Nat.  Hist.  Survey  of  Jlinnosota,  Final  Kept.,  Vol.  IV,  1899,  p.  246. 

cQeol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  pp.  272,  273,  and  283. 

''  Report  on  the  geology  of  the  Rainy  Lake  region:  Geol.  and  Nat.  Hist.  Survey  of  Canada,  1889, 
F.  pp.  21-3.5  et  seq.  Geology  of  the  Rainy  Lake  region:  Am.  Jour.  Sci.,  .Sd  series,  Vol.  XXXIII, 
1887.  p.  477. 


ELY  GREENSTONE. 


159 


general  statement  in  the  preface  of  the  final  volume  of  the  Minnesota  survey, 
Winchell  abandoned  the  use  of  the  term  Coutchiching,  for  his  studies 
showed  that  he  could  not  in  this  district  include  any  definite  series  there- 


Fig.  2. — Reproduction  of  sketch  by  A.  Winchell,  showing  the  intricate  relationship  between  the  granite  of  Burntside 

Lake  and  the  amphibole-schists. 


160  THE  VERMILION  IRON -BEARING  DISTRICT. 

under".  The  rocks  that  were  first  included  under  this  term  can  be  shown 
to  l^e  to  a  large  extent  formed  by  metamorphism  from  the  Ely  gi-een- 
stones  as  above  described.  The  remaining  portion  has  been  formed  by 
metamorphism  of  sediments  of  Lower  Huronian  age,  as  described  in 
Chapter  IV.  Should  Lawson's  name  Coutchiching  be  applied  to  the 
amphibole-  and  mica-schists  lying  between  the  granites  of  the  district  and 
the  rocks  that  have  been  intruded  by  the  granite,  we  should  have  included 
under  this  term  two  series  of  rocks  which,  though  possessing  the  same 
schistose  characters,  are  demonstrably  of  different  age,  both  as  regards 
their  initial  period  of  formation  and  their  period  of  metamorphism. 
The  use  of- the  name  Coutchiching  is  not  wan-anted  in  connection  with  the 
rocks  of  the  Vermilion  district  of  Minnesota,  and  Lawson's  insistence '' on  the 
presence  of  a  series  of  rocks  in  this  district  comparable  to  his  supposed 
Coutchiching  series. is  explainable  only  as  due  to  his  unfamiliarity  with  the 
district. 

The  character  of  the  metamorphism  involved  in  the  change  of  the 
greenstone  of  the  Archean  from  a  massive  rock  to  a  predominantly  schistose 
rock  of  a  different  mineralogic  character  might  be  made  a  matter  of  question 
by  some  who  wish  to  classify  metamorphic  rocks  into  those  produced  by 
contact  action  and  those  produced  by  regional  metamorphism.  The  ag'ents, 
however,  in  both  cases  are  the  same.  Thej-  are  heat,  pressure,  and  water, 
and  whether  these  agents  owe  their  activity  to  the  intrusion  of  an  igneous 
mass  of  rock  or  to  orogenic  movement  is  merely  a  matter  of  detail.  In 
the  present  instance  the  field  relations  of  the  greenstones  to  the  metamor- 
phic rocks  and  the  granite  show  that  the  metamorphism  of  the  greenstones 
accompanied  the  intrusion  of  the  granite.  Hence,  as  this  was  the  prime 
agent  in  their  'production,  they  hav6  been  classed  under  contact  metamor- 
phic products.  Yet  while  these  schistose  rocks  may  well  have  been  pro- 
duced by  the  intrusion  of  igneous  masses  that  caused  recrystallization  of 
their  already  partiall}-  altered  original  minerals,  nevertheless  essentially  the 
same  chemical  constituents  are  present  in  them  now  as  were  present  in 
them  formerly.    The  rocks  have  merely  been  recrystallized  under  pressure. 

"Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV,  1S99,  pp.  14  and  15. 
''Geol.  and  Nat.  Hi.-^t.  i^arvey  of  >Iinne.«ota,  V)y  N.  H.  Winohell;  Final  Rept.,  Vol.  IV.     Review- 
by  A.  C.  Lawsiin,  Am.  .Toiir.  Sci.,  4th  series,  Vol.  IX,  1900,  p.  1.51. 


ELY  GREENSTONE.  161 

This  has  resulted  in  the  formation  of  minerals  with  higher  specific  gravity" — 
hence  such  as  occupy  less  space — and  produced  an  arrangement  of  these 
minerals  which  makes  them  conform  to  the  law  of  the  production  of 
secondary  minerals  in  schists,  as  explained  by  Leith.''  Moreover,  the  great 
additional  space  within  this  portion  of  the  earth's  crust  which  was  required 
by  the  intrusion  of  the  granite  has  been  partly  supplied  by  this  very  change 
of  the  preexisting  greenstones  into  related  rocks  that  occupy  less  volume. 
In  their  formation,  pressure  has,  of  course,  been  a  very  important  factor; 
hence  the  more  frequent  occurrence  among  them  of  schistose  forms  of 
rocks. 

CONTACT  EFFECT  OF  GABBRO  ON  ELY  GREENSTONE. 

In  three  areas  the  Archean  EI3"  greenstone  lies  in  juxtaposition  with 
gabbro  of  Keweenawan  age.  •  A  contact  of  the  greenstone  with  the  gabbro 
occurs  east  of  Disappointment  Lake,  in  sees.  26  and  35,  T.  64  N.,  R.  8  W. 
Here  the  anticline  of  Ely  greenstone  has  been  cut  across  on  its  east  side  by 
the  gabbro.  Another  contact  occurs  in  sees.  1  and  2,  T.  64  N.,  R.  6  W.,  at 
the  southwest  side  of  Grobbemichigamma  Lake.  From  sec.  25,  T.  65  N., 
R.  5  W.,  eastward  through  sees.  30,  29,  28,  and  27,  T.  65  N.,  R.  4  W.,  we 
find  an  Archean  anticline  which  is  not  in  contact  with  the  gabbro,  being 
separated  from  it  by  a  minimum  distance  of  perhaps  20()  paces  and  a 
maximum  distance  of  half  a  mile.  The  greenstone  has  been  metamorphosed 
by  the  gabbro,  although  it  has  not  been  affected  nearl}^  so  extensively  as  it 
is  where  it  is  in  contact  with  the  granite.  Macroscopically  no  great 
difi"erence  can  be  observed  between  the  metamorphosed  and  the 
unmetamorphosed  greenstones.  They  are  in  all  cases  massive  rocks, 
and  the  metamorphosed  portions  appear  to  have  essentially  the  same  ' 
characters  as  the  remaining  unmetamorphosed  portions,  although  the 
former  weather  somewhat  more  readily  than  the  latter  and  have  a 
rusty  brown  color. 

The  effect  of  the  gabbro  on  the  greenstone  in  producing 
metamorphosed  rocks  can  be  best  seen  in  exposures  on  the  west  side 
of    Gobbemichigamma     Lake    in    the     sections    above    mentioned,    on 


«  Metamorphism  of  rocks  and  rock  flovvage,  by  C.  R.  Van  Hise:  Bull.  Geol.  Soc.  Am.,  Vol.  IX, 
1897,  p.  291. 

6  Manuscript. 

MON  XLV — 03 11 


162  THE  VERMILION  IKON-BEARING  DISTRICT. 

the  south  flank  of  the  Twin  Peaks  ridg-e.  Here  the  texture  of  the 
greenstone  is  characteristically  ophitic,  a  secondary  hornblende  taking 
the  place  of  the  original  pyroxene.  When  metamorphosed  by  the  galjbro 
we  find  the  ophitic  texture  perfectly  preserved  with,  however,  a  large 
quantity  of  biotite  as  a  secondary  pi'oduct.  This  biotite  has  accumulated 
around  the  edges  of  the  hornblende  between  the  hornblende  and  the  feld- 
spar, and  is  especially  concentrated  along  evident  shearing  planes  where 
normally — that  is,  in  the  greenstone  unaft'eeted  b}^  the  gabbro — one  would 
find  a  large  amount  of  chlorite  derived  from  the  hornblende.  With  rocks 
like  the  above  there  is  associated  anotlier,  showing  the  ophitic  texture 
poorly  preserved  and  with  brownish-green,  massive  hornblende  constituting 
most  of  the  rock,  and  with  hyperstheue  occurring  in  more  or  less  porphyritic 
areas.  This  hypersthene  is  very  fresh  and  seems  to  be  a  product  of  the 
action  of  the  gabbro  on  the  greenstone.  In  other  cases  ophitic  textured 
greenstones  seem  to  contain  a  very  much  larger  amount  of  a  brownish- 
green  hornblende  and  magnetite  than  these  green.stones  normally  contain, 
and  in  this  instance  the  large  quantity  of  magnetite,  and  possibly  also  the 
brown  hornblende,  is  assumed  to  be  due  to  the  action  of  the  gabbro.  In 
general,  there  are  produced  from  the  greenstone,  by  metamorphism  of  the 
gabbro,  rocks  which  contain  a  large  percentage  of  biotite  and  varying 
quantities  of  hypersthene  and  magnetite.  As  a  result  of  their  mineralogic 
character  such  rocks  have  a  rusty-brown  color,  and  the  texture,  although 
distinctly  ophitic,  is  inclined  to  become  granulai'  as  the  new  minerals 
increase  in  quantity.  These  rocks  disintegrate  much  more  readily  than  do 
the  greenstones. 

RELATION  OF  EliT  GREENST01S1E  TO  ADJACEXT  FORISIATIONS, 

The  relations  of  the  gi-eenstone  complex  to  the  adjacent  formations 
have  already  been  briefly  stated,  but  will  be  recapitulated.  Wherever  the 
gi'eenstone  complex  is  in  contact  with  any  sediments  all  the  larger  masses 
lie  above  and  are  infolded  in  it.  When  these  sedimentaries  are  normal 
clastic  deposits  the}^  lie  above  and  contain  numerous  fragments  of  the  green- 
stones, showing  that  the  gi-eenstone  complex  is  the  older  formation.  When 
the  gi'eenstones  lie  next  to  other  igneous  rocks  they  are  found  to  be 
penetrated  by  them.  Hence  all  of  the  relations  of  the  greenstone  complex 
to  the  various    adjacent  formations   prove   its    greater    age.     A   detailed 


ELY  GREENSTONE.  163 

description  of  some  of  the  contacts  of  the  various  formations  of  the  district 
with  the  greenstones,  in  which  their  relations  to  one  another  will  be  given, 
will  be  found  under  the  discussion  of  these  formations. 

ECONOMIC  VAIiUE  OF  THE  ELY  GREENSTONE. 

The  Ely  greenstone  rocks  of  the  Vermilion  district  are  suitable  for 
building  stones,  especially  for  foundations,  for  which  the  unworked  stones 
can  be  used.  They  are  very  tough,  and  there  is  hardly  sufficient  demand 
for  stonework  to  warrant  their  being  cut  and  used  for  the  superstructure 
of  buildings,  even  if  their  color  were  suitable.  Their  color  is,  however, 
uniformly  so  dark  that  they  would  rarely  be  used  for  other  portions  of 
buildings  than  foundations  and  trimmings.  This  stone  is  eminently  adapted 
for  use  as  road  material,  as  there  is  an  inexhaustible  supply,  and  it  is  gener- 
ally so  distributed  that  it  can  be  obtained  for  use  on  existing  roads  at  small 
cost. 

Occasionally  the  discovery  of  bodies  of  magnetite  ore  in  the  green- 
stones is  announced.  The  greenstone  contains  large  quantities  of  magnetite 
as  an  essential  constituent.  This  occurs,  however,  disseminated  through 
the  rock  in  very  small  particles,  which  make  up  an  exceedingly  small 
percentage  of  the  total  mass.  While  the  occurrence  of  the  ore  bodies 
reported  has  in  no  case  been  verified,  it  would  not  be  at  all  surprising  should 
iron-oxide  bodies,  of  very  limited  extent,  however,  really  be  found.  The 
explanation  of  their  occurrence  would  be  similar  to  that  of  the  occurrence  of 
almost  identical  iron-oxide  masses  in  the  gabbro ;  that  is,  they  are  the  result 
of  processes  of  segregation  from  the  basic  magma.  It  is  not  believed,  how- 
ever, that  any  such  bodies  that  may  be  found  would  prove  to  be  of  commer- 
cial value.  The  iron  oxide  occurring  in  minute  quantities  scattered  through 
the  greenstone  contains  titanium — it  is  a  titaniferous  magnetite — and  the 
probability  is  that  any  ore  bodies  found  in  this  greenstone  would  likewise 
consist  of  titaniferous  magnetite.  They  would  then  correspond  in  .  their 
chemical  composition,  as  well  as  in  their  mode  of  origin,  to  the  ore  bodies  in 
the  gabbro.  Moreover,  since  the  processes  of  liquation  and  fractional  crys- 
tallization, as  the  result  of  which  such  bodies  are  formed,  would  be  most 
fully  carried  out  in  those  cases  where  the  rocks  remain  under  essentially 
the  same  conditions  of  temperature  and  pressure  for  a  great  length  of  time, 
we  should  naturally  expect  to  find  the  largest  bodies  of  oxide  in  the  coarsest- 
grained  rocks.     Hence,   continuing  the  comparison  of  tlie  bodies  of  oi'e 


164  THE  ^'ERMILION  IRON-BEARING  DISTRICT. 

which  one  would  be  likely  to  find  in  the  Archean  greenstones  with  those  in 
the  adjacent  Keweenawan  gabbro,  we  see  that  the}'  would  probably  be 
very  much  smaller  than  those  in  the  gabbro,  since  the  greenstones  with 
which  they  would  occur  are  of  much  finer  grain  than  the  gabbro.  More- 
over, as  the  size  of  the  mass  of  magma  has  an  important  bearing  upon  the 
rate  of  cooling,  we  may  say  that  the  larger  the  mass  of  magma  the  larger 
the  ore  body.  For  this  reason  also  we  should  expect  to  find  smaller  bodies 
of  oxides  in  the  greenstone  than  in  the  gabbro. 

In  many  portions  of  the  world  very  important  ore  bodies  containing 
other  metals  than  iron  are  found  associated  with  rocks  of  essentially  the 
same  composition  as  those  forming  the  greenstone  complex.  The  question 
may  well  be  asked,  What  are  the  chances  of  finding  silver,  nickel,  and 
cobalt  ores,  to  mention  some  of  the  most  important,  in  association  with 
these  greenstones?  I  would  answer  that  there  is  practically  no  chance. 
In  other  regions  the  ores  mentioned  occur  as  contact  deposits  which  owe 
their  occurrence  to  the  intrusion  of  rocks  allied  to  these  greenstones  into 
younger  rocks,  the  deposits  being  found  in  fissures  occurring  within  the 
younger  rock,  within  the  older  i-ock,  partially  in  both,  or  along  the  contact 
between  the  two.  Although  these  greenstones  cover  a  broad  area,  yet, 
since  they  are  themselves  the  oldest  rocks,  we  can  not  expect  to  find  such 
deposits  in  them  in  very  large  quantity  unless  they  occur  within  the 
greenstones  themselves  as  the  product  of  processes  of  segregation — pro- 
cesses which,  as  has  been  intimated,  may  have  given  origin  to  certain  iron- 
ore  deposits  reported  to  occur  in  them,  but  whose  existence  remains 
unverified.  Winchell  refers  to  the  occurrence  of  a  gold-bearing  quartz 
vein  in  the  following  words:" 

At  the  west  end  of  Long  Lake,  SW.  i  SW.  i  sec.  30,  T.  63  [N.,  R.]  13  [W.].  is  a 
conspicuous  display  of  quartz  and  granite,  the  former  carrying  gold.  An  average 
sample  selected  from  the  dump,  assayed  by  F.  F.  Sharpless,  gave  §8.64.  Some  casual 
working  has  been  done  on  this  vein,  and  numerous  assays  show,  according  to  the 
statement  of  Mr.  Mcintosh,  one  of  the  owners,  an  average  of  over  $10  per  ton.  The 
vein  is  traceable  about  an  eighth  of  a  mile,  a  little  north  of  east,  with  an  irregular 
width  reaching  a  maximum  of  about  SO  feet.  It  accompanies  a  granite  dike.  The 
ore  is  not  abundant,  but  is  in  irregular  streaks  in  the  quartz. 

Thus  far  no  gold-bearing  veins  which  have  paid  Tor  the  working  of 
them  have  Ijeen  found  in  the  Vermilion  district. 


«N.  H.  Winchell,  Geol.  and  Nat  Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV,  1899,  p.  258. 


ELY  GREENSTONE.  165 


INTERESTING  LOCALITIES. 


Under  this  heading  there  will  be  found  special  descriptions  of  certain 
localities  where  the  rocks  show  especially  well  some  of  the  characters 
already  described,  or  for  other  reasons  are  considered  worthy  of  more 
detailed  mention  than  has  been  made  of  them  in  ^Jreceding  pages.  These 
descriptions  of  localities  may  not  perhaps  be  read  by  the  general  reader, 
but  it  is  hoped  may  be  useful  to  future  students  of  the  geology  of  the 
district  who  may  wish  to  verify  the  statements  herein  made,  and  who  would 
therefore  desire  to  visit  some  of  these  places. 

Some  of  the  best  places  at  which  the  general  characters  of  the  Ely 
greenstone  may  be  studied,  are  on  the  hills  near  Ely.  These  hills  are 
very  nearly  bare,  and  numerous  exposures  of  the  greenstone  may  be  found 
on  them.  The  ellipsoidal  parting  is  well  shown  in  exposures  in  the  cut 
on  the  south  side  of  the  railroad  track  west  of  the  station,  and  can  be 
seen  in  numerous  places  on  the  bare  hills  between  Ely  and  Long  Lake. 
Ajuygdaloidal  structure  is  also  very  commonly  present.  No  spherulites 
were  observed  here,  although  these  are  abundant  on  the  high  hills  due 
north  of  Ely,  on  the  north  side  of  Long  Lake.  On  the  hill  west  of  the 
town  and  south  of  the  water  tank  ellipsoidal  parting,  with  peripherally 
arranged  amygdules,  may  be  observed,  and  at  one  place  not  very  far  from 
the  road  leading  up  to  the  cemetery  the  transition  from  the  ellipsoidally 
parted  portion  into  the  nonellipsoidal  greenstone  can  be  seen.  The 
greenstones  on  this  hill  are  cut  by  dikes  of  granite-porphyry.  Just 
northeast  of  the  Methodist  church  on  the  east  side  of  Ely  numerous  basic 
dikes  are  found  cutting  the  greenstones.  About  a  mile  and  a  half  south 
of  Ely  is  a  bare  ridge,  bordered  on  the  north  and  south  by  considerable 
depressions,  on  which  there  are  many  exposures  that  show  the  amygdaloidal 
and  ellipsoidal  characters  of  the  greenstones.  Irregular  lines,  which  seem 
to  represent  flowage  lines,  run  through  these  rocks  in  many  places.  This 
ridge  offers  a  fairly  good  place  for  the  study  of  the  volcanic  characters  of 
the  greenstones.  It  must  be  noted,  however,  that  the  greenstones  in  this 
ridge  have  been  extremely  altered  and  in  many  places  are  more  or  less 
completely  schistose,  and  would  now  be  spoken  of  as  amphibole-schists. 
This  alteration  is  due  to  the  intrusion  of  the  Giants  Range  granite,  which  is 
^present  in  these  rocks  in  numerous  dikes,  and   which  occurs    in    mass  a 


166  THE  VERMILION  IRON-BEARING  DISTRICT. 

short  distance  to  the  south.  These  exposures,  therefore,  besides  offering 
opportunity  for  a  study  of  the  general  characters  of  the  greenstone,  are 
very  favorable  for  a  study  of  the  metamorphism  of  the  greenstone  into  the 
amphibole-mica-schists,  which,  as  seen  in  isolated  exposures,  in  many  cases 
offer  little  evidence  of  their  derivation  from  the  greenstones. 

POSSIBLE   TUFFS   ASSOCIATED   WITH    THE    GREENSTONES. 

Reference  has  already  been  made  to  the  fact  that,  associated  with  the 
greenstone,  we  not  uncommonly  find  masses  of  tuffaceous-looking  rocks, 
which,  since  they  show  no  characters  clearly  indicative  of  sedimentation, 
have  been  included  with  the  greenstones  as  interbedded  tuff  deposits. 
Included  in  this  category  are  deposits  occumng  at  the  following  places: 

North  200  paces,  west  1,950  paces  from  southeast  corner  sec.  17,  T. 

62  N.,  R  13  W. 

North  600  paces,  west  1,000  paces  from  southeast  corner  sec.  30,  T. 

63  N.,  R.  11  W. 

North  1,930  paces,  west  1,000  paces  from  southeast  corner  sec.  3,  T. 
63  N.,  R.  13  W. 

North  2,000  paces  from  southeast  corner  sec.  3,  T.  63  N.,  R.  13  "W. 

On  east  shore  of  large  lake  in  T.  62  N.,  R.  14  W.,  just  south  of  the 
east-west  section  line  between  sees.  25  and  36. 

Another  area  is  that  occurring  1,650  paces  north  of  southeast  comer 
sec.  20,  T.  63  N.,  R.  10  W.  Here  the  tuffaceous  rock  has  been  sheared 
and  where  most  schistose,  with  the  schistosity  striking  N.  70°  E.,  black 
jasper  has  been  infiltrated. 

EVIDENCES   OF   VOLCANIC   CHARACTER. 

Beginning  about  1,500  paces  north  of  the  southeast  corner  sec.  3,  T. 
62  N.,  R.  12  W.,  and  extending  south  along  the  section  line  to  the 
quarter  post,  there  are  numerous  exposures  of  dark-gray  to  green  rocks 
which  have  irregular  lines  running  through  them,  and  possess  a  more 
or  less  perfectly  preserved  amygdaloidal  and  ellipsoidal  structure.  The 
lines  referred  to  are  thought  to  be  flowage  lines.  These,  in  connection 
with  the  other  structures  mentioned,  seem  to  be  fair  proof  of  the  A'olcanic 
character  of  the  rocks.  These  volcanics  are  penetrated  by  dikes  of  granite 
which  vary  in  size  from  very  small  ones  an  inch  or  more  in  width  to  some 
haviug  a  width  that  is  measurable  by  yards.     The  rocks  themselves  are 


ELY  GREENSTONE.  167 

impei-fectly  schistose,  and  may  well  be  called  ampliibole-  and  mica- schists. 
The  rocks  in  this  locality  represent  one  of  the  passage  phases  between  the 
normal  g-reenstones  on  the  one  hand  and  the  ampliibole-  and  mica-schists 
on  the  other,  which,  when  close  to  the  main  mass  of  the  granite,  show 
none  of  the  volcanic  structures  that  enable  then-  original  character  to  be 
easily  determined  here.  This  passage  from  the  normal  greenstones 
through  the  amygdaloidal  and  ellipsoidal  green  schists  to  the  normal  schists 
next  to  the  granite  can  be  seen  still  better  just  north  of  the  quarter  post 
between  sec.  19,  T.  62  N.,  R.  12  W.,  and  sec.  24,  T.  62  N.,  R.  13  W., 
and  also  along  the  quarter  line  in  the  sou-theast  quarter  of  sec.  24,  T.  62 
N.,  R.  13  W.  At  this  last  locality  we  pass  from  the  granite  into  an  area 
in  which  the  schists  and  granites  are  most  intricately  mixed.  The  schists 
occasionally  still  possess  an  imperfect  amygdaloidal  structure.  To  the 
north  we  soon  pass  from  very  schistose  greenstones  to  those  which  are 
onl}^  slightly  schistose  and  in  which  amygdaloidal  and  ellipsoidal  structures 
are  well  developed. 

The  ellipsoidal  structure  and  the  presence  of  spherulites,  found  most 
frequently  in  association  with  these  ellipsoids,  have  been  referred  to  as 
common  features  of  the  greenstones.  Greenstones  possessing  both  of  these 
characters  occur  very  commonly  in  large  exposures  throug-hout  sec.  10 
and  the  west  half  of  sec.  11,  T.  63  N.,  R.  10  W.  They  can  be  very  clearly 
seen  at  a  number  of  the  exposures  here,  especially  on  one  about  200 
paces  north  and  1,000  paces  west  of  the  southeast  corner  of  sec.  10, 
T.  63  N.,  R.  10  W.  Here  the  greenstone  is  separated  into  large  ellipsoids 
and  the  spherulites  are  arranged  in  concentric  circles  within  the  ellip- 
soids. The  smallest  spherulites  occur  near  the  periphery  of  the  ellipsoids; 
the  largest,  3  inches  in  diameter,  occur  nearer  the  center.  These  large 
spherulites  show  their  radial  structure  in  sections  on  the  weathered 
surfaces  of  the  rock.  Some  of  them  now  consist  of  a  chloritic  mineral 
having  a  dark-green  color  and  a  silky  luster.  In  most  cases  they  are 
lighter  colored,  and  the  mineral  constituting  them  is  feldspar.  The  inter- 
ference of  the  spherulites  with  one  another  in  the  process  of  growth  is 
very  prettily  shown  in  many  places.  Very  rarely  are  they  perfectly 
round.  In  most  cases  they  have  interfered  with  each  other,  and  while  in 
some  places  nearly  perfect  spherulites  may  be  observed,  they  are  most 
commonlv  irregularly  rounded  and  surrounded  by  others  which  have  the 


168  THE  VERMILION  IRON-BEARING  DISTRICT. 

form  of  segments  of  circles.  It  is  evident  that  they  did  not  all  begin  to 
form  at  just  the  same  time  or  else  their  rate  of  growth  was  not  just  the 
same,  for  if  their  origin  were  simultaneous  and  their  rate  of  growth  equal 
we  would  get  in  cross  section  through  such  masses  a  structure  resembling 
that  of  a  honeycomb.  These  spherulitie  greenstones  are  both  fine  and 
coarse  grained,  the  ellipsoidal  and  spherulitie  portions  being  continuous 
with  the  nonellipsoidal  and  nonspherulitic  greenstones.  It  would  seem 
that  the  ellipsoidal  and  spherulitie  portions  of  the  greenstones  represent  the 
surface  of  greenstone  lavas  which  are  to  be  considered  as  effusive  sheets  or 
flows.     These  greenstones  are  cut  by  dikes  of  granite-porphja-y. 

One  mile  north  of  North  Twin  Lake,  at  the  northeast  corner  of  sec.  12, 
T.  63  N.,  R.  10  ^Y.,  these  ellipsoidal  spheruhtic  greenstones  continue  from 
south  of  Jasper  Lake  in  almost  continuous  exposures  eastward  along  the 
south  section  line  of  sec.  6,  T.  63  N.,  R.  9  W.,  almost  as  far  east  as  the  south 
quarter  post  of  that  section.  Here  the  ellipsoids  reach  a  diameter  of  3  to 
4  feet  and  some  of  them  are  solid  masses  of  spherulites,  each  spherulite 
showing  its  radial  structure  very  beautifully  on  the  weathered  surface. 

About  400  paces  north,  100  paces  west  from  the  southeast  corner  of 
sec.  35,  T.  62  N.,  R.  14  W.,  interbedded  amygdaloidal  and  ellipsoidal  basalts 
occur.  They  are  all  somewhat  schistose.  As  we  continue  southward 
studying  the  exposures,  we  find  that  they  show  an  increasing  degree  of 
metamorphism.  They  finally  become  amphibole-  and  mica-schists  and 
gneisses,  and  but  for  the  presence  of  the  elongated  ellipsoids  and  the 
amvgdules,  filled  with  chlorite  and  pinkish  quartz,  one  could  not  be  sure, 
from  the  field  study,  of  theii-  igneous  origin.  The  rocks  in  this  locality 
resemble  in  a  striking  degree  the  crystalline  schists  occurring  in  the 
vicinity  of  Bone  Lake",  in  the  Crystal  Falls  district  of  Michigan. 

It  has  been  stated  repeatedly  that  certain  of  the  greenstones  possess 
an  amygdaloidal  structure,  and  that  with  these  tuffs  are  associated.  These 
facts  have  been  cited  as  evidence  of  the  volcanic  nature  of  the  greenstones. 
Such  associated  and  presumably  interbedded  tuifs  and  amygdaloidal  and 
porphyritic  greenstones  are  very  well  exposed  upon  the  west  and  north- 
west slopes  of  a  high  hill  in  the  northwest  quarter  of  sec.  19,  T.  64  N., 
R.  10  W.     Here  tlie  greenstones,  which  are  both  fine  and  coarse  grained, 

"The  Crystal  Falls  ■iron-bearing  district  of  Michigan:  Mon.  U.  S.  Geol.  Survey  Vol.  XXX VI, 
1899,  pp.  148-152. 


U.   S.   GEOLOGICAL   SURVEY 


MONOGRAPH    XLV      PL.    V 


m; 


(^) 


A.  AMYGDALOIDAL   GREENSTONE   (M  ETABASALT). 

B.  MAGNETITIC   CHERT,   SHOWING    POSSIBLE   LINES  OF   FALSE   BEDDING. 

The  bands  of  brilliant  red  jasper  commonly  show  lines  similar  to  these,  but  without  the  irregularities  indicative  of  false  bedding. 


ELY  GREENSTONE.  169 

are  dotted  with  abundant  amygdules.  In  some  instances  amygdules  are 
scattered  over  large  areas.  In  other  instances  they  are  collected  chiefly 
along  certain  lines  which  run  about  northeast.  The  individual  lava  flows 
could  not  be  distinguished.  The  amygdules  are  now  (ival,  with  the  long 
axes  of  the  ovals  parallel  with  the  schistositv  of  the  rocks,  wliich  trends 
N.  60°  E.  The  amygdules  range  from  very  small  ones  to  others  which 
are  3  inches  long  and  three-fourths  of  an  inch  across  the  shortest  diameter. 
The  accompanying  illustration,  PI.  Y,  .4,  is  a  reproduction  of  the  polished 
surface  of  one  of  these  metamorphosed  amygdaloidal  basalts.  The  rock 
is  now  an  amphibole-schist,  and  but  for  the  presence  of  the  amygdules 
and  its  associations  in  the  field  it  would  not  be  recognized  as  a  basic  lava. 
These  greenstones  are  cut  by  dikes  of  granite  as  well  as  by  narrow  dikes 
of  basic  rock. 

The  ellipsoidally  parted  greenstones  are  well  exposed  in  the  west  half 
of  sec.  17,  east  half  of  sec.  18,  northeast  quarter  of  sec.  19,  and  north- 
west quarter  of  sec.  20,  T.  63  N.,  R.  9  W.  The  ellipsoidal  parting  and 
amygdaloidal  characters  of  these  rocks  show  clearly  their  identity  with 
the  Ely  greenstone  in  other  parts  of  the  district.  The  ellipsoids  vary  in 
size  considerably,  and  the  matrix  between  them  varies  from  one-half  inch 
to  2  inches  in  thickness.  This  matrix  has  been  silicified  to  a  consider- 
able extent,  and  in  places  appears  very  much  like  a  black  chert.  The 
rocks  have  been  extensively  metamorphosed.  This  metamorphism  is  prob- 
abl}'  due  largely  to  the  effects  of  the  Keweenawan  gabbro,  which  at 
present  is  separated  fi'om  the  greenstone  by  the  width  of  the  Kawishiwi 
River.  Formerly,  however,  the  gabbro  unquestionably  overlaid  the  green- 
stone to  the  north  of  the  river.  The  greenstone  is  cut  by  a  number  of 
dikes  of  basic  rocks  varying  in  width  from  a  few  inches  to  30  feet.  The 
dikes  are  dolerite  and  camptonite  (f). 

METAIVIORPHISM  OF  THE  GREENSTONES. 

The  greenstones  of  the  Vermilion  district  have  been  extremely 
metamorphosed  by  the  intrusion  of  younger  rocks,  as  well  as — perhaps 
chiefly — by  the  intense  folding  to  which  they  have  been  subjected.  The 
oldest  rocks,  the  Ely  gi-eeustones,  have  naturally  been  most  metamor- 
phosed, since  they  have  been  subjected  not  only  to  all  of  the  folding 
which  affected  the  younger  rocks,   but  to  previous  metamorphic  action. 


170  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  following  description  of  an  area  on  the  portage  between  Wind  and 
Moose  lakes  makes  clear  the  difficulty  which  one  experiences  in  attempt- 
ing" to  discriminate  between  these  o'reenstones  and  some  of  the  rocks  that 
are  associated  with  and  derived  from  them.  The  rock  on  the  north  side 
of  the  high  ridge  overlooking  Wind  Lake  is  undoubtedly  a  green  schist 
derived  from  the  Ely  greenstone.  It  is  broken  by  diagonal  joints  running 
in  two  directions,  lines  bisecting  the  acute  angles  of  the  rhomboids  formed 
by  the  intersection  of  these  joints  being  parallel  to  the  strike.  The  minute 
diagonal  fractures  have  been  largely  cemented  by  some  material,  as  is 
shown  by  ridges  that  run  along  the  weathered  surfaces.  The  rock  is  veined 
with  white  quartz,  but  there  is  no  definite  banding  except  that  produced 
by  this  material  which  has  filled  the  fractures.  There  is  no  appearance 
whatever  of  pebbles  in  the  rock.  The  schistosity  is  A-ery  strongly  marked. 
Along  the  schistosity  there  are  numerous  fine  quartz  veins  and  some 
fractures  filled  by  other  materials,  so  that  the  rock  shows  a  fairly  distinct 
banding  parallel  ti;>  the  schistosity. 

South  of  this  green  schist  there  is  a  peculiar  rock  ■\^'nicll  shows  ver}' 
fine  banding  along  the  schistosity,  as  does  the  green  schist  abo^'e  described. 
This  banding  does  not  seem  to  be  due  to  secondary  cementation,  but  appar- 
ently results  from  the  mashing  of  originally  heterogeneous  material.  On 
weathered  surfaces  of  this  rock  that  lie  transverse  to  the  schistosity  there  are 
obscure  roundish  or  elliptical  spots  having  their  long  directions  parallel 
with  the  trend  of  the  schistosit)'.  These  roundish  spots  are  presumably 
mashed  pebbles.  They  are  very  numerous,  and  increase  in  distinctness  as 
one  passes  southward  from  the  contact,  and  within  a  distance  of  30  t)r  40 
feet  the  rock  assumes  a  distinctly  conglomeratic  apj^earance.  When  the 
rock  is  split  the  pebble-like  masses  are  seen  to  be  g-reatly  extended  in 
the  direction  of  the  schistosity.  These  masses  are  several  times  longer 
in  the  direction  of  the  dip  than  along  the  strike,  and  from  two  to  ten  times 
as  broad  (along  the  strike)  as  they  are  thick.  In  these  respects  tiie  rock 
is  identical  with  the  remarkable  schist  conglomerates  of  Vermont  described 
by  Hitchcock"  in  1860. 

In  the  conglomeratic  rock  at  Wind  Lake  the  regular  system  of  fractures 
spoken  of  as  occurring  in  the  mass  first  described — the  green  schist  adja- 
cent to  the  conglomerate — are  not  present,  at  least  with  any  such  regularity, 

"(ienlojry  of  Veniinnt,  Vol.  I,  lS(il,  iip.  28-42. 


ELY  GREENSTONE.  171 

although  there  are  irregular  diagonal  fractures  wider  apart.  It  seems  as  if 
there  had  been  distributire  movement  about  each  pebble-like  area,  so  that 
in  this  wa}^  readjustment  occurred  largely  in  the  softer  matrix-like  material 
rather  than  in  the  regular  fashion  shown  in  the  homogeneous  rock  to  the 
north.  These  facts  lead  to  the  conclusion  that  the  southern  part  of 
the  ledge  is  decomposed,  much  mashed,  detrital  material  derived  from  and 
resting  upon  homogeneous  schistose  material.  Here  it  is  rather  difficult  to 
draw  the  dividing  line  between  the  Ely  greenstone  and  the  conglomerate 
derived  from  it.  At  other  places,  where  the  mashing  has  not  been  so 
extensive  and  where  the  pebbles  are  far  more  distinctive,  and  especially 
where  the  rock  contains  pebbles  of  jasper  and  granite  as  well  as  of  green- 
stone, the  line  of  separation  can  be  much  more  readily  drawn. 

Indeed,  a  little  farther  southeast,  along  this  same  trail — that  is,  on  the 
Wind  Lake-Moose  Lake  trail — a  perfectly  distinct  conglomerate  occurs 
which  consists  predominantly  of  greenstone  j^ebbles  and  bowlders,  some  as 
large  as  3  feet  in  diameter,  but  which  contains  many  associated  granite 
pebbles.  From  the  repetition  of  different  zones  of  conglomerate  occurring 
on  this  portage  it  would  seem  that  the  rocks  had  been  Yerj  intricately 
folded  here  and  that  the  several  different  zones  are  reall}-  a  single  zone 
repeated  as  a  result  of  the  close  folding. 

In  the  southeast  quarter  of  sec.  18,  T.  62  N.,  R,  13  W.,  exposures  are 
pretty  numerous,  and  one  finds  here  good  opportunities  for  studying  the 
transition  from  the  greenstone,  with  ellipsoidal  parting  here  and  there,  to 
the  amphibole-mica-schists.  These  schists  occur  in  their  best,  most  typical 
development  in  close  proximity  to  the  Giants  Range  granite,  which,  as  has 
already  been  stated  in  detail  elsewhere,  is  considered  to  be  the  cause  of 
their  existence.  In  all  such  cases  the  farther  one  goes  from  the  contact  the 
less  altered  are  the  greenstones  found  to  be,  whereas  the  nearer  the  contact 
the  more  intricately  are  the  greenstones  cut  by  the  granite  dikes,  and  the 
more  nearly  they  assume  the  characters  of  typical  schists.  The  same 
observations,  pointing  toward  the  production  of  the  schists  from  the  green- 
stones by  the  intrusion  of  the  granite,  may  be  made  at  many  places  along 
the  boundary  between  the  granite  and  the  greenstone,  extending  from 
sec.  31  northeastward  through  sees.  32  and  29  into  sec.  28,  all  in  T.  62  N., 
R.  13  W.  At  numerous  places  along  this  line  the  hornblende-schists  still 
possess  the    ellipsoidal   and   less  commonly  the    amygdaloidal   characters 


172  THE  VERMILION  IRON-BEARING  DISTRICT. 

of  the  unaltered  greenstones.  In  some  places  the  vesicles  are  half  an 
inch  in  diameter,  and  their  character  is  undoubted.  Most  commonly  these 
are  filled,  and  the  amygxlules  are  very  prominent,  though  in  other  cases 
the  amvgdules  have  been  weathered  out.  Not  infrequently  faint  lines 
of  somewhat  lighter  color  than  the  main  mass  of  the  rock  may  be  observed 
crossing  the  exposures  diagonally  to  the  schistosity.  These  lines  seem  to 
be  most  common  where  the  amygdaloidal  structure  is  most  noticeable,  and 
they  appear  to  be  the  traces  of  flow  structm-e  in  the  lavas. 

In  the  southeast  quarter  of  sec.  16,  T.  64  N.,  R.  9  W.,  just  north- 
west of  the  southwestern  end  of  Newfound  Lake,  there  is  a  high  hill, 
bare  over  a  great  portion  of  its  surface,  consisting  of  the  Ely  greenstone 
cut  through  and  through  by  the  granite  of  Basswood  Lake.  In  some  places 
the  two  rocks  are  present  in  about  equal  amount,  though  in  general  the 
greenstone  predominates.  The  greenstone  has  been  so  much  metamoi'phosed 
by  the  granite  that  it  would  be  more  accurately  classed  as  an  amphibole- 
schist.  In  many  places  the  structural  peculiarities  of  the  Ely  greenstone 
may  be  observed.  This  hill  affords  a  good  opportunity  for  study  of  the 
relations  between  the  granite  and  the  schist,  and  especially  for  obser-s-ing 
the  general  characters  of  the  amphibole-schists.  Moreover,  the  fact,  already 
stated,  that  the  greenstone  does  not  consist  of  a  single  rock,  liut  of  a  com- 
plex, is  clearly  shown  by  the  presence  of  a  coarse,  much  metamorphosed 
dike  of  dolerite  which  cuts  the  schist  and  includes  it  and  which  in  turn  is 
cut  by  the  younger  granite. 

SECTION  111 —SOUDAN  FORMATION. 

The  Soudan  formation,  a  division  of  the  Archean,  lies  above  and  is 
mainly  younger  tlian  the  Ely  greenstone.  It  contains  the  important  iron- 
ore  deposits  of  the  district  and  is  well  developed  and  exposed  at  the  town 
of  Soudan,  where  are  located  also  some  of  the  most  important  mines  of 
the  district. 

OCCURRENCE  AND   CHARACTER. 

DISTRIBUTION. 

The  iron-bearing  formation  begins  10  miles  east  of  the  western  limit  of 
the  district  as  outlined  in  this  report,  and  caia  be  traced  eastward  for  many 
miles,  the  easternmost  occurrence  seen  being  a  very  limited  exposure  south 
of  Moose  Lake,  in  sec.  4,  T.  63  N.,  R  9  W.     However,  the  same  formation 


SOUDAN  FORMATION.  173 

occurs  just  north  of  the  boundary  in  Ontario,  and  is  known  to  continue 
northeastward  for  many  miles  within  Canadian  territory. 

The  Soudan  formation  has  its  greatest  development  in  the  western  part 
of  the  district,  the  most  prominent  areas  extending  from  Tower,  on  Ver- 
milion Lake,  in  T.  62  N.,  R.  15  W.,  on  the  west,  to  Fall  and  Garden  lakes, 
in  T.  63  N.,  R.  11  W.,  just  a  few  miles  east  of  the  well-known  town  of  Ely, 
on  the  east. 

The  formation  is  most  notably  exposed  in  areas  lying  about  midway 
between  the  north  and  south  limits  of  the  district.  At  Tower  and  Soudan 
it  underlies  broad  areas  and  forms  the  prominent  topographic  features 
known  as  Tower,  Lee,  and  Soudan  hills,  and  Chester,  or  Jasper,  Peak. 
Other  fairly  large  areas  occur  in  a  belt  just  north  and  east  of  Ely,  in  sec. 
25,  T.  63  N.,  R.  12  W.,  and  in  sec.  30,  T.  63  N.,  R.  11  W.  North  and  south 
of  and  between  the  areas  mentioned,  the  formation  underlies  rather  narrow 
belts  trending  east-northeast  to  west-southwest.  Each  of  these  belts  is  made 
up  of  a  series  of  narrow  bands  of  the  iron  formation,  interbedded  in  some 
cases  with  small  quantities  of  fragmental  rocks  and  intimately  associated 
with  the  Ely  greenstone  and  the  late  intrusives,  which  cut  through  both 
the  Ely  greenstone  and  the  Soudan  formation.  As  shown  on  the  maps 
in  the  accompanying  atlas,  some  of  these  belts,  especially  those  near  the 
center  of  the  western  part  of  the  district,  can  be  followed  for  a  number  of 
miles  east  and  west;  one  was  traced  for  16  miles.  Others  are  very  much 
shorter,  having  been  traced  for  only  a  few  miles,  and  these  small  areas 
grade  down  to  those  which  are  mere  patches,  a  few  inches  or  feet  across 
and  a  few  feet  or  paces  in  extent — that  is,  along  the  strike.  However,  it  is 
believed  that  all  of  these,  from  the  largest  to  the  smallest,  with  the  excep- 
tion possibly  of  certain  small  vein-like  masses  which  will  be  mentioned 
later,  are  parts  of  one  general  formation,  now  separated  from  one  another 
by  folding  and  erosion. 

A  glance  at  the  maps  will  show  that  the  areal  distribution  of  the 
Soudan  formation  is  closely  connected  with  that  of  the  Ely  greenstone. 

EXPOSURES. 

One  would  be  inclined  to  think,  judging  from  the  resistant  nature  of 
the  rocks  constituting  the  iron  formation,  that  it  would  be  well  exposed 
throughout  the  district.  Such  is,  however,  very  far  from  the  case.  Except 
in  a  few  places,  notably  at  Tower,  Lee,  and  Soudan  hills  and  Jasper  Peak; 


174  THE  VERMILION  IRON-BEARING  DISTRICT. 

in  sec.  25,  T.  63  N.,  R  12  W.;  sec.  30,  T.  63  N.,  R.  11  W.;  .sees.  3  and 
4,T.  61  N.,  R.  15  W.;  sees.  7  and  8,  T.  62  N.,  R.  14  W.;  sec.  6,  T.  62  N;, 
R.  14  W.,  and  sec.  1,  T.  62  N.,  R.  15  W.;  the  exposures  are  very  poor.  But 
the  fact  that  the  exposures  of  the  iron  formation  are  scarce  and  small  in  some 
localities  does  not  necessarily  mean  that  the  formation  at  .such  places  is  not 
now  or  may  not  become  in  the  future  of  very  great  economic  importance. 
For  example,  the  immensely  valuable  iron  deposits  at  Ely,  extending  fi-om 
the  Chandler  mine  on  the  west  to  the  Savoy  on  the  east,  occur  where  jasper 
exposures  are  remarkably  few. 

In  the  belts  traced  through  the  district  the  exposures  are  small  and 
discontinuous,  both  along  and  across  the  strike,  and  this  would  make  it 
impossible  to  trace  out  any  horizons  in  the  iron-bearing  formation,  even  if 
they  could  be  determined,  but,  owing  to  the  uniformity  of  the  formation, 
such  horizons  can  not  be  fixed.  In  making  use  of  the  accompanying  maps 
it  should  be  clearly  understood  that  the  colors  or  patterns  indicate  merely 
that  the  iron  formation  has  been  found  in  the  areas  so  colored.  The  limits 
fixed  do  not  necessarily  imply  that  the  area  is  underlain  wholly  by  the 
formation,  for,  as  has  ah'eady  been  intimated  above,  in  many  instances 
exposures  of  greenstone,  equally  as  numerous  and  as  large,  occur  in  these 
belts  in  intimate  association  with  the  jaspers.  With  these  occur  also 
younger  intrusives,  which  cut  through  them.  In  fact,  it  is  impracticable  to 
say  which  of  these  two  kinds  of  rock  preponderates  in  many  of  such  areas. 
The  iron-bearing  formation  certainly  does  preponderate  in  a  number  of  the 
well-known  areas  which  will  at  once  occur  to  those  acquainted  with  the 
district — for  example,  on  Tower  and  Lee  and  Soudan  hills,  Jasper  Peak, 
ridge  in  sec.  25,  T.  63  N.,  R.  12  W.,  and  ridge  in  sec.  30,  T.  63  N.,  R.  11  W. 
The  above  statement  will  hold  true,  however,  on  the  whole,  for  the  smaller 
belts.  The  belts  outlined  represent  the  possible  ore-bearing  areas,  and 
such  areas  having  once  been  outlined  as  closely  as  possible  by  the  geolo- 
gist, it  then  remains  for  the  mining  companies  to  make  more  detailed  studies 
of  them  than  it  was  possible  for  the  members  of  the  Sm-vey  to  make  in  the 
limited  time  at  their  disposal. 

In  the  course  of  the  field  work  the  occurrence  of  the  iron  formation 
has  been  reported  from  various  localities,  but  search  failed  to  reveal  expo- 
sures in  these  places.     It  is  highly  possible  that  in  the  future  other  areas 


SOUDAN  FORMATION.  175 

than  those  which  are  outhned  on  the  accompanying  maps  will  be  found, 
but  it  can  be  confidently  stated  that  they  will  in  all  cases  be  small  and 
presumably  of  very  sliglit  importance. 

TOPOGRAPHY.  , 

The  amount  of  the  iron  formation  in  the  district  is  relatively  so  small 
that  it  can  scarcely  be  said  to  have  had  any  great  effect  upon  the  general 
topography.  In  those  areas  where  it  is  best  developed  it  does  influence 
the  topography  very  materially.  As  the  result  of  the  resistant  character 
of  the  jasper,  which  is  the  predominant  rock  in  the  formation,  strongly 
marked  hills  persist  where  it  is  present  in  large  quantity.  Of  these,  the 
most  striking  are  Lee  and  Tower  hills,  Jasper  or  Chester  Peak,  the  hill 
forming  the  prominent  northeast  point  of  Stuntz  Bay,  and  the  hills  in  sec. 
7,  T.  62  N.,  R  14  W.,  and  in  sec.  4,  T.  61  N.,  R  14  W.;  also  the  prominent 
ridge  extending  through  sec.  25,  T.  63  N.,  R  12  W.,  and  sec.  30,  T.  63  N., 
R.  11  W.  In  the  various  belts  containing  the  iron  formation  the  jasper 
very  commonly  occupies  minor  prominences,  the  low  ground  between  being 
occupied  presumably  by  the  associated  greenstones  and  sediments. 

STRUCTURE. 

The  iron-bearing  Sondan  formation  being  the  oldest  sedimentary 
formation  in  the  district  has  been  subjected  to  all  of  the  orogenic  move- 
ments which  have  occuri'ed  in  the  district  since  its  deposition.  Since  there 
were  several  of  these  movements,  and  since  the  forces  producing  them 
were  verv  intense,  the  formation  has  been  most  intricately  folded.  It  is 
indeed  difficult  to  describe  or  represent  the  intricacy  of  the  folding  which 
it  exhibits  npon  nearly  every  exposure  of  any  size. 

On  a  large  scale  the  formation  has  been  folded  into  anticlines  and 
synclines,  and  its  structure  is  now  shown  to  a  certain  extent  by  the  topog- 
raphy. Thus,  for  example,  the  prominent  hills — Lee,  Tower,  and  Soudan — 
are  great  anticlines  with  minor  synclines  and  anticlines  s;iperimposed  upon 
them,  whereas  Jasper  Peak  is  situated  at  the  end  of  a  syncline,  and  on  its 
western  and  southern  sides  shows  very  prettily  the  jasper  folded  into  a 
series  of  rolls  pitching  a  little  to  the  east  of  north.  It  is  also  highly  prob- 
able that  some  of  the  east- west  trending  belt's  of  the  iron  formation  are  to 
be  considered  as  synclines  of  jasper  infolded  in  the  older  greenstone. 
Upon  the  more  prominent  anticlines  and  synclines  numerous  minor  folds 


176  THE  VERMILION  IRON-BEARING  DISTRICT. 

are  superimposed,  giving  the  intricac}"  of  structure  already  referred  ti^.  It 
is  of  interest  to  note  in  connection  A\-itli  this  remarkably  close  folding  of 
the  jasper — some  of  the  bands  are  actually  folded  upon  themselves  within 
a  radius  scarcely  greater  than  the  width  of  the  belt — that  the  jasper  for  the 
most  part  has  not  been  very  much  fractured.  This  is  very  clearly  indica- 
tive of  the  great  depth  at  which  this  formation  lay  at  the  time  the  folding- 
took  place.  This  close  folding'  without  fracture  can  be  explained  only  by 
assuming  that  the  rocks  were  under  such  great  pressure  that  they  acted 
practically  as  ^jlastic  bodies.  North  of  Fall  Lake  the  close  folding  of  the 
jasper  is  shown  in  one  place  where  bands  4  to  5  inches  in  width  have  been 
turned  so  sharply  that  the  two  ends  are  now  only  1  foot  apai't,  and  here 
the  jasper,  usually  considered  a  very  brittle  substance,  shows  no  indica- 
tions of  fractures,  but  has  comported  itself  as  a  viscous  material. 

PI.  VI,  A  and  B,  reproduced  from  sketches  made  by  W.  N.  MeiTiam  in 
the  field,  from  actual  exposiu-es  near  Soudan,  shows  very  well  the  extreme 
intricacy  of  the  folding. 

Both  the  longitudinal  and  the  cross  folding  of  the  iron  formation  is 
composite;  that  is,  superimposed  upon  the  major  folds  in  each  direction  are 
folds  of  the  second  order,  and  upon  these  are  folds  of  the  third  order,  and  so 
on  down  to  minute  plications.  The  pressure  has  been  so  great  as  to  give  all 
variety  of  minor  folds,  including  isoclinal  and  fan  shaped.  Moreover,  these 
varieties  of  folds  may  be  seen  almost  equally  well  on  a  ground  plan  or  on 
a  vertical  cross  section.  The}-  are  beautifully  shown  at  various  places 
about  Tower  and  Ely,  but  perhaps  the  most  extraordinarily  complex  folding- 
seen  is  that  at  the  west  end  of  the  large  island  in  the  east  part  of  Emerald 
Lake.  Figs.  A  and  B  of  PL  VI,  which  are  upon  the  whole  representative 
of  the  district,  show  that  the  folding,  notwithstanding  the  extremely  brittle 
character  of  the  rock,  was  accomplished  without  major  fracture.  The 
deformation,  therefore,  is  deformation  in  the  zone  of  rock  flowage,  and  no 
better  instance  is  known  to  the  writer  of  this  kind  of  earth  movement. 
Frequently  a  solid  belt  of  jasper  is  bent  back  upon  itself  within  a  radius 
of  its  own  width  Avith  no  sign  of  fracture. 

Though  the  folding  is  so  complex  as  to  give  even  fan-shaped  folds,  the 
turns  are  ordinarily  round  rather  than  angled,  thus  differing  from  those 
acute-angled  folds  frequently  seen  in  the  Menominee  district.  The  round- 
ness of  the  folds  is  well  illusfrnted  in  the  figures. 


U.  S    GEOLOGICAL   SURVEY 


MONOGRAPH    X  LV   PL   VI 


rA) 


ilEN  &  CO.LlTM.N  1 


FOLDED  AND  B  RECCIATED  JASPER   OFTHE   SOUDAN    FORMATION. 


SOUDAN  FORMATION.  177 

The  Vermilion  district,  therefore,  appears  to  be  one  of  the  best  regions 
in  the  world  to  illustrate  complex  folds,  or  folding  in  two  directions  at  right 
angles  to  each  other,  and  the  formation  that  best  exhibits  this  folding-  is 
the  Soudan.  This  is  due  to  the  very  marked  banding  of  that  formation, 
by  means  of  which  the  position  of  bedding  is  readily  determined,  and  to 
the  fact  that  for  the  most  part  it  does  not  take  on  any  secondary  structure. 
Furthermore,  it  frequently  is  found  in  contact  with  the  Ely  greenstone, 
which  also  gives  the  pitch  of  the  cross  folds. 

This  remarkably  complex  folding  partly  explains  the  distribution  of 
the  Soudan  formation  with  reference  to  the  Ely  greenstone.  Naturally, 
where  the  formation  is  thick  it  is  found  along  the  border  of  the  greenstone. 
However,  since  upon  the  major  folds  are  superimposed  secondarj^  and 
tertiarj^  folds,  numerous  patches  of  the  jasper  occur  in  the  greenstone. 
Moreover,  because  of  the  cross  folding,  these  patches  may  be  very  narrow 
at  one  place,  widen  out  very  rapidly  so  as  to  make  a  thick  formation,  and 
again  narrow.  When  the  extraordinary  complexity  of  this  folding  is 
understood  one  has  only  to  premise  an  erosion  extending  to  different 
depths  in  the  Soudan  formation  before  the  Lower  Huronian  was  deposited 
in  order  to  see  how  in  the  greenstone  the  jasper  may  range  in  size  or  extent 
from  patches  a  few  feet  in  width  and  length,  to  the  great  continuous  forma- 
tion about  Tower  and  Ely.  Moreover,  such  premise  fully  explains  the 
extraordinary  variation  in  width  of  the  jasper  belts  at  some  places  and  their 
persistency  and  uniformity  at  others. 

Occasionally  there  is  associated  with  the  iron  formation  and  inter- 
banded  with  the  jasper  some  bands  of  slaty  material.  In  places  the 
amount  of  this  slaty  material  is  so  great  that  where  folding  has  •  taken 
place  a  slaty  cleavage  has  developed  in  these  layers.  This  cleavage, 
however,  does  not  pass  through  the  bands  of  ii'on  oxide  or  chert.  These 
bands  with  the  slaty  cleavage  afford  excellent  oj^portunities  for  makino- 
observations  upon  the  relations  of  cleavage  to  the  direction  of  pressure. 
In  these  bands  this  development  of  slaty  cleavage  is  seen  to  obey  the  laws 
of  slaty  cleavage,  as  explained  by  Van  Hise."  PI.  VII,  a  representation  of 
a  specimen  taken  from  the  folded  jaspers,  shows  this  cleavage  so  clearly 
that  textual  explanation  is  scarcely  needed. 

"Principles  of  North  American  pre-Cambrian  geology,  by  C.  R.  Van  Hise:  Sixteenth  Ann. 
Kept.  U.  S.  Geol.  Survey,  Pt.  I,  1896,  pp.  363-369. 
MON  XLV — 03 12 


178  THE  VERMILION  IRON-BEARING  DISTRICT. 

Even  wliere  the  bands  of  slaty  material  in  the  rocks  are  not  more  than 
one-fourth  of  an  inch  across,  the  slaty  cleavage  is  perfectly  developed  and 
stops  abniptly  at  the  adjacent  more  biittle  cherty  material.  Thus  we  have 
in  tills  phase  of  the  iron  formation  numerous  layers  showing  good  slaty 
cleavage  alternating  with  others  in  which  it  is  absent.  The  slaty  cleavage 
is  in  such  ])osition  in  reference  to  the  plications  as  to  show  that  it  developed 
normally  to  the  pressm-e.  The  lack  of  parallelism  of  the  cleavage  upon 
opposite  sides  of  the  folds  beautifully  illustrates  the  pnnciple  that  on 
anticlines  the  cleavage  on  opposite  sides  of  folds  diverges  downward  and  on 
synclines  converges  downward.  These  alternating  slate  and  jasper  bands 
are  well  shown  in  the  so-called  "Burnt  Forties"  adjacent  to  Vermilion  Lake. 

While  usually  deformation  has  taken  place  without  fracture,  the  jasjjer 
is  sometimes  brecciated.  We  sometimes  find  very  pretty  "reibungs"  or 
friction  breccia  formed  of  the  jasper  fragments  cemented  together  by  vein 
quartz.  Not  uncommonly  such  a  brecciated  zone  occurs  near  the  base  of 
the  iron  formation,  between  it  and  the  lower -lying  greenstones,  and  is  thus 
clearly  the  result  of  movement  along  the  plane  separating  the  two  kinds  of 
rocks.  In  such  instances,  the  jasper,  being  the  more  brittle  of  the  two 
rocks,  forms  the  angular  to  parti}' rounded  fragments  of  the  breccia,  whereas 
the  greenstone,  in  some  cases  at  least,  is  found  to  have  been  forced  in 
between  the  jasper  fragments  and  to  play  the  part  of  a  matrix  cementing 
tlie  l)reccia- together  (PI.  VI,  C).  The  plane  of  brecciation  being  more  open, 
has  been  es})ecially  favorable  for  the  free  movement  of  underground  water. 
Similar  brecciated  zones  at  the  base  of  the  jasper — that  is,  between  it  and 
the  greenstones — due  to  movements  along  this  plane,  occur  near  the  west 
end  of  Emerald  Lake,  just  north  of  the  international  boundary,  on  the 
point  that  projects  eastward  from  the  south  shore  of  this  lake.  Consequent 
upon  this  lirecciation  there  has  been  infiltration  of  various  substances, 
especially  of  quartz  and  iron  oxide  subsequent  to  the  formation  of  the 
breccia,  which  also  tends  to  cement  the  fragments  together  and  likewise  to 
tllseolor  the  rock. 

On  the  east  end  of  Lee  Hill,  on  the  south  side  of  the  old  North  Lee 
mine,  there  is  a  brecciated  zone  in  which  the  above-mentioned  conditions 
can  be  observed.  It  is  further  ver}'  noticeable  here — and  the  same  thing 
may  be  seen  at  otliei-  places — that  the  fragments  are  frequently  cemented 
together  by  very  pure  hematite,  and  when  there  were  favorable  cavities  of 
sufficient  size  subordinate  bodies  of  very  high-grade  ore  were  deposited. 


U.   8.   GEOLOGICAL  SURVEY 


MONOGRAPH  XLV      PL.   VII 


FOLDED   JASPER    AND    SLATE,    SHOWING    SLATY    CLEAVAGE    DEVELOPED    IN    SLATE    BANDS. 


SOUDAN  FORMATION.  179 

In  spite  of  the  intricacy  of  the  folding  of  the  iron  formation,  it  has  been 
possible  to  determine  that  in  general  the  axes  of  the  major  folds  strike  east- 
northeast  to  west-southwest,  the  clearest  instance  of  such  a  large  fold  beina: 
the  syncline  at  Ely.  The  dip  of  the  axial  plane  of  these  folds  appears 
almost  without  exception  to  be  steep  to  the  north,  indicating  that  the  close- 
ness of  the  folding  has  been  very  great  and  overturns  are  common  results 
of  this.  It  is  interesting  to  find  that  this  axial  plane  lias  been  subject  to 
torsional  movement,  as  in  the  case  of  the  plane  of  the  Ely  syncline.  This 
upon  the  west  end  dips  to  the  north,  but  underground  explorations  at  the 
east  end  show  that  it  has  here  a  reversed  dip  to  the  south. 

PETROGRAPHIC  CHARACTERS. 
MACROSCOPIC  CHARACTERS  OF  THE  FRAGMENTAL  PORTION   OF    THE  SOUDAN  FORMATION. 

The  Soudan  formation  may  be  divided  into  a  fragmental  series  of 
sediments  and  into  the  iron  formation  jDroper,  whose  origin  is  not  distinctly 
clastic. 

The  clastic  portion  of  the  formation  will  be  described  first,  for  the 
reason  that  it  always  underlies  the  iron  formation  j^roper.  The  very  few 
occurrences  of  these  clastic  sediments  are  so  widely  separated  from  each 
other  areally  that  it  is  impossible  to  say  that  they  all  belong  to  the  same 
beds,  although  they  everywhere  bear  the  same  relation  to  the  iron  forma- 
tion. And  in  this  connection  it  must  be  borne  in  mind  that  there  is  a 
possibility  that  the  different  bands  of  the  iron  formation  are  not  of  exactly 
the  same  age,  but  represent  stages  of  deposition  of  slightly  different  age, 
although  all  of  the  same  general  period.  The  sediments  of  the  clastic 
division  of  the  iron  formation  are  grayish  green  and  black  in  color,  and 
consist  of  a  conglomerate  at  the  base,  grading  upward  into  the  finer-grained 
deposits.  The  conglomerate  lies  next  to  the  greenstone,  and  consists — both 
matrix  and  pebbles — of  the  material  derived  therefrom  and  some  pebbles 
of  vein  quartz.  The  finer  sediments  are  chiefly  finer  material  of  the  same 
character,  derived  from  the  same  source.  The  exception  to  this  statement 
would  be  certain  soft,  black,  graphitic  slates  which  are  found  associated  with 
the  jaspers.  Such  may  be  observed,  for  example,  upon  the  westernmost 
exposm-es  of  Lee  Hill,  just  back — that  is,  north — of  the  houses  of  Tower. 
A  somewhat  similar  graphitic  slate  is  found  on  the  southern  slope  of  Soudan 
Hill,  about  200  yards  northeast  of  No.  12  shaft,  which  is  northwest  of  the 


180  THE  VERMILION  IRON-BEARING  DISTRICT. 

Minnesota  Iron  Company's  warehouse.  The  presence  of  these  graphitic 
slates  with  the  iron  foi'mation  probably  accounts  for  the  large  masses  of 
gi-aphitic  rock  on  the  twelfth  level  in  No.  8  shaft  at  Soudan.  There  is  a 
mass  of  this  graphitic  rock  10  feet  long  and  from  6  to  8  feet  thick 
completely  lying  in  the  soap  rock.  It  was  cut  in  a  third  dimension  for  11 
feet,  and  how  much  farther  it  may  extend  in  this  direction  is  unknown. 
This  graphitic  rock  was  tested,  it  is  said,  by  Mr.  John  H.  Eby,  sometime 
mining  engineer  of  the  Minnesota  Iron  Company,  who  reported  it  as 
graphite. 

These  clastic  sediments  are  interbedded  with  the  jasper  and  other 
materials  constituting  the  iron  formation  proper.  Toward  the  iron 
formation  the  bands  apparently  become  more  frequent,  and  the  clastic 
sediments  decrease  in  amoimt,  and  there  is  thus  a  gradual  transition  into 
the  iron  formation  proper. 

The  lower  clastic  portion  of  the  fonnation  is  l^y  no  means  characteristic. 
It  is  rarely  present,  and  when  present  is  A-ery  thin.  In  one  place  about 
40  feet  of  sediments,  chiefly  conglomerate,  were  seen,  but  the  exposures 
were  so  poor  that  it  was  impossible  to  tell  whether  the  beds  were  duplicated 
bv  folding  or  not.  It  is  possible  that  some  fairly  wide  areas  separating 
jasper  exposm-es  from  greenstone  exposures  may  be  underlain  by  the 
clastic  formation,  but  this  is  not  probable,  for  tlie  practical  absence  of  the 
formation,  winch  is  fairly  resistant  tlu'oughout  the  district,  shows  that  it 
must  have  been  very  subordinate.  Yet,  in  spite  of  its  subordinate  position 
quantitively,  this  clastic  portion  of  the  formation  is  of  great  stratigraphic 
importance,  as  no  matter  at  how  few  places  it  has  been  found  or  how  thin 
it  may  be,  it  nevertheless  is  clear  proof  of  a  very  important  change  in 
conditions,  marking  the  transition  from  the  period  of  volcanic  acti^^ity  in 
which  the  greenstone  had  its  origin  to  the  period  of  sedimentary  deposition ' 
in  which  the  Soudan  formation  was  laid  down. 

MICROSCOPIC   CHARACTERS   OF   THE    FRAGMENTAL    PORTION   OF   THE    SOUDAN 

FORMATION. 

The  conglomerate  and  noiTnal  fine-grained  sediments,  belonging  for  the 
most  part  below  the  iron-bearing  formation  proper,  show  nothing  under  the 
microscope  which  is  worthy  of  detailed  description.  The  conglomerates 
are  clearly  recognizable  in  the  field  as  elastics,  and  luider  the  microscope 


SOUDAN  FORMATION.  181 

one  can  distinguisli  the  extremely  altered  greenstone  fragments  and  the 
matrix  derived  from  the  greenstones,  both  consisting  now  largely  of 
actinolite,  chlorite,  and  epidote,  with  quartz.  As  the  sediments  get  finer 
the  clastic  characters  disappear  as  the  result  of  the  extreme  alteration,  and 
one  can  only  surmise  the  mode  of  origin  of  these  rocks  by  their  intimate 
association  with  and  gradations  from  the  coarse  elastics,  and,  when  no 
gradation  is  visible,  by  the  presence  of  banding  and  false-bedding  lines. 
The  microscopic  examination  shows  these  sediments  to  be  made  up  of 
chlorite,  actinolite,  epidote,  sericite,  sphene,  quartz,  carbonaceous  material 
(graphite),  and  some  iron  oxides,  in  various  proportions,  so  that  these 
metamorphosed  slates  vary  from  absolutely  black,  greasv-feeling,  graphitic 
slates  to  dark-green  and  fairly  light  greenish-gray  rocks.  The  graphitic 
slates  consist  essentially  of  graphite  and  quartz  in  exceedingly  fine  grains 
and  in  some  cases  in  very  small  quantity.  In  one  specimen  the  place  of 
the  quartz  seemed  to  be  taken  by  feldspar,  which  is  altered  to  sericite,  so 
that  the  sediment  consists  of  graphite  and  altered  feldspar. 

MACROSCOPIC    CHAEACTERS    OF    THE    IRON-BEARING    FORMATION    PROPER. 

The  iron-bearing  formation  proper  (that  is,  that  portion  in  which  the 
ore  bodies  occur)  consists  of  cherts  of  various  colors — green,  white, 
yellow,  black,  and  purplish — red  jasper,  carbonate-bearing  chert,  slaty  rock, 
showing  in  some  cases  intimate  association  with  the  clastic  formation  proper, 
griinerite-magnetite-schist,  hematite,  magnetite,  and  some  pyrite.  To  the 
formation  as  a  whole  the  miners  and  prospectors  apply  the  name  "jasper," 
although  only  a  portion  of  it  falls  strictly  under  this  designation.  These 
various  kinds  of  rock  occur  in  bands  of  varying-  thickness,  rarely  exceeding 
5  or  6  inches,  and  commonly  in  extremely  thin  laminaj.  Usually  the 
individual  bands  appear  to  be  homogeneous.  Occasionally  there  is  a 
banding  within  the  bands,  which  is  due  to  the  arrangement  of  the  mineral 
constituents.  In  one  place  such  a  banding  simulated  the  false  bedding 
of  normal  elastic  sediments  (PI.  V,  5). 

The  alternate  bands  of  material  of  different  color  combined  with  the 
complicated  folding  make  the  formation  a  very  striking  object,  which  on 
exposures  almost  always  attracts  the  attention  of  the  traveler,  even  if  he  is 
not  accustomed  to  closely  noticing  rocks. 


182  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  hematite,  besides  being  interbanded  with  the  other  materials,  also 
occurs  very  frequently  in  masses  of  variable  size  and  shape.  These  consti- 
tute the  ore  deposits  of  the  district,  and  will  be  considered  under  a  separate 
head.  The  bands  are  not  arranged  in  any  definite  order,  but  alternate  with 
one  another,  giving  a  very  regular  ribbon  or  banded  structure.  All  the 
colored  cherts  have  the  white  as  a  base,  the  difference  in  color  being  due 
chiefly  to  the  presence  of  the  iron,  either  in  the  form  of  magnetite,  hematite, 
or  limonite,  or  as  a  combination  of  these.  The  black  cherts,  frequently 
called  black  jasper  or  hungry  jasper,  always  contain  a  large  quantity  of 
magnetite,  to  which  they  owe  their  color.  The  brilliant-red  jasper  owes  its 
color,  as  is  well  known,  to  the  thin  transparent  plates  and  minute  specks 
of  blood-red  hematite.  The  color  of  the  brown  cherts  is  due  to  the  limonite. 
The  colors  of  the  other  varieties — gray,  brown,  and  ocherous  yellow — depend 
on  the  mixtures  of  the  above  oxides  or  of  their  alteration  products.  Of 
somewhat  rarer  occurrence  is  the  slightly  greenish  and  grayish  chert,  which, 
although  subordinate  in  quantity,  is  important  in  reference  to  the  genesis 
of  the  iron-bearing  formation.  This  chert  contains  a  considerable  amount 
of  iron  carbonate  and  griinerite,  to  which  it  owes  its  color.  These  chert 
bands  become  yellow  and  brown  on  weathering,  on  account  of  the  for- 
mation of  ocher  by  the  decomposition  of  the  carbonate  and  griinerite.  The 
hematite  and  magnetite  bands  associated  with  the  cherts  are  very  rarely 
pure.  On  examining  them  it  will  be  found  in  almost  every  case  that  the 
hematite  bands  contain  varying  percentages  of  magnetite,  and  vice  versa. 
With  these  of  course  one  is  always  sure  to  find  a  variable  quantity  of 
quartz.  Iron  pyrite  is  mixed  with  these  various  rocks  in  small  amounts, 
but  it  is  not  known  to  occur  in  large  quantity  in  this  district. 

The  most  intimate  relationship  exists  between  the  various  above- 
mentioned  members  of  the  iron-bearing  formation  proper.  Gradual  transi- 
tion from  one  into  another  may  be  traced.  Near  the  west  end  of  Tower 
Hill,  in  following  the  strike  of  the  rocks,  one  finds  the  jasper  becoming  less 
brilliantly  colored  and  grading  with  continuous  exposures  into  the  black 
magnetitic  jasper.  The  iron  formation  has  been  folded,  and  as  a  consequence 
is  traversed  by  more  or  less  frequent  fractures.  These  fractures  have  been 
filled  by  veins  of  quartz  which  run  transverse  to  the  banding  in  the  iron 
formation.  The  smaller  cracks  have  very  commonly  been  filled  by  iron 
oxide  that  is  in  all  respects  identical  with  that  occurring  interbanded  with 


SOUDAN  FORMATION.  183 

the  jaspers.  The  secondary  nature  of  the  oxide  that  fills  the  cracks  is 
indisputably  shown  by  this  occurrence,  and  is  strongly  indicative  of  the 
secondary  origin  of  that  in  the  bands,  the  two  probably  being  of  contem- 
porary formation.  In  studying  the  formation  it  was  noted  that  two  series  of 
cracks  had  been  formed  in  the  jaspers,  the  older  having  been  filled  with 
vein  quartz  and  the  younger  with  hematite. 

THE   IKOS   ORBS. 

The  uj^per  part  of  the  Soudan  formation  is  in  a  strict  sense  the  ore- 
bearing  portion.  Indeed,  this  is  the  iron-bearing  formation  which  has  given 
to  the  Vermilion  district  its  great  economic  importance,  since  from  this  have 
been  derived  great  quantities  of  the  high-grade  ore  which  has  assisted 
materially  in  making  the  Lake  Superior  region  the  greatest  single  factor  in 
the  development  of  the  iron  industry  of  the  United  States  and  of  the  world. 
The  ores  of  the  Vermilion  district  comprise  several  varieties — massive, 
graniilar  hematite,  specular  hematite,  and  insignificant  amounts  of  mag- 
netite and  limonite.  There  are,  of  course,  also  all  kinds  of  mixtures  of 
these,  showing  gradational  phases  from  one  variety  to  the  other. 

The  predominant  ore  is  an  exceedingly  hard,  massive,  granular,  steel- 
blue  hematite.  The  specular  ore  occurs  locall}'^  in  small  masses.  The 
magnetite  is  obtained  only  in  small  quantities,  and  is  intimately  associated 
with  the  hematite.  Occasionally  small  bodies  of  magnetite  ore  are  found, 
not  large  enough  to  be  of  special  value,  or  to  warrant  an  attempt  to  obtain 
a  grade  of  magnetite  ore.  Such  occurrences  are  very  exceptional.  The 
■  limonite  is  very  subordinate,  occurring  only  associated  with  the  hematite. 
There  seems  to  be  a  general  misapprehension  as  to  the  character  of  the  ore 
in  the  Chandler  mine  at  Ely,  the  greatest  producer  of  the  district.  It  is 
very  commonly  spoken  of  as  a  soft  ore.  This  is,  however,  purely  a  relative 
term,  in  this  case  depending  upon  the  brecciated  condition  of  the  ore,  which 
enables  it  to  be  won  with  less  drilling  and  with  much  less  expenditure 
of  high  explosives  than  is  required,  for  instance,  in  the  Minnesota  Iron 
Company's  mine  at  Tower.  The  ore  is  found  in  an  extremely  brecciated 
condition  by  the  miners,  and  this  brecciation  is  taken  advantage  of  and 
really  increased  by  the  method  of  mining  employed.  As  a  result  of  this 
more   or  less  finely  brecciated   condition  the  ore  is  obtained  to  a  great 


184  THE  VERMILION  IRON-BEARING  DISTRICT. 

extent  by  the  use  of  picks.  The  fragments  of  tlie  breccia  are,  however,  the 
same  hard  ore  that  occurs  in  the  other  mines  of  the  district,  but  as  a  result 
of  the  brecciation,  a  great  deal  of  very  finely  comminuted  ore  is  associated 
with  the  larger  fragments  and  occurs  between  them.  The  cause  of  the 
brecciated  condition  of  this  ore  will  be  discussed  more  at  length  under  the 
heading  "  Ore  deposits." 

The  ores  of  the  district  are  rendered  impure  by  various  mechanical 
mixtures  of  quartz,  calcite,  chlorite,  iron  pyrites,  native  copper,  the  oxide 
of  copper  (cuprite),  and  the  carbonates  (malachite  and  azurite).  These 
copper  ores  are  present  in  very  small  quantity,  however,  and  are  of  chief 
interest  on  account  of  the  fact  that  this  occui-rence  of  these  minerals  in 
association  with  the  ores  at  Soudan  is  the  first  recorded  from  the  Lake 
Superior  region."  Mr.  Pengilly  informed  the  writer  that  native  copper  had 
been  found  in  the  Chandler  mine  several  years  prior  to  its  known  occur- 
rence at  Soudan.  Quartz,  calcite,  chlorite,  and  pyrite  occur  locally,  but  in 
considerable  quantity,  and  as  a  result  large  quantities  of  ore  are  thrown 
away  in  the  attempt  to  get  rid  of  these  impurities.  Good  hand  specimens 
showing  these  minerals  can  always  be  obtained  from  the  dump  piles  and 
even  the  stock  piles  of  the  Soudan  mines.  The  minerals  occur  along  the 
walls  of  vugs  of  various  sizes  which  exist  in  the  ore  bodies. 

The  iron  content  of  the  Vermilion  iron  ores,  computed  froin  cargo 
analyses  made  during  1899,  varies  from  60.47  to  67.37  per  cent,  and 
averages  about  63.7  per  cent.  The  phosphorus  content  varies  from  0.04  to 
0.131  per  cent,  and  averages  about  0.057  per  cent.  The  silica  content 
varies  from  2.55  to  7.67  per  cent,  and  averages  4.78  per  cent.  The  water 
content  varies  from  1.04  to  7.956  per  cent,  and  averages  about  5.50  per 
cent.  The  ore  bodies  are  of  such  importance  that  their  origin,  the 
occurrence  of  the  ore  in  them,  etc.,  will  be  considered  in  detail  under 
separate  headings. 

The  i)hysical  character  of  the  ores  is  such  as  to  make  them  much  desired 
by  the  smelters  for  admixture  with  the  softer,  finer-grained  ores.  The  ores 
are  hard  and  are  obtained  in  large  pieces  and  run  tln-ough  crushers  and 

"The  occurrence  of  copper  minerals  in  hematite  ore,  Montana  mine,  Soudan,  Minnesota;  descrip- 
tion of  the  occurrence,  tiy  J.  H.  Kby;  study  of  tlie  minerals,  by  V.  P.  Berkey:  Trans.  Lake  Superior 
Mining  Institute,  Vol.  IV,  IS'JG,  pji.  G9-79. 


SOUDAN  FORMATION. 


185 


broken.  The  following  table,  made  by  Mr.  R.  B.  Green,  in  1898,  at  the 
Minnesota  Iron  Company  laboratory  at  Two  Harbors,  Minn.,  shows  the 
coarseness  of  the  Chandler  and  Pioneer  ores  of  Ely.  Ores  from  Soudan 
are,  perhaps,  even  coarser. 

Percentage  of  ore  from  Chandler  and  Pioneer  mines,  Ely,  Minn.,  that  passes  through 

screens  of  specified  mesh  per  inch. 

[Determined  in  natural  state  after  taking  from  cars  and  drying.] 


Kind  of  ore. 


Chandlei'  ore  (28  cargoes) 

Long  Lake  ore,  Chandler  mine  (13  cargoes) 
Pioneer  ore  (23  cargoes) 

Pilot  ore,  Chandler  mine  (2  cargoes) 


Does  not  pass 
8  meshes. 

8  meshes  to 
20  meshes. 

20  meshes  to 
100  meshes. 

82.05 

9.14 

5.86 

76.99 

10.94 

8.08 

77.50 

11.00 

7.  08  i 

65.65 

13.  93 

11.44 

100  meshea. 

2.94 
3.98 
4.42 
8.97 


MICROSCOPIC    CHARACTERS    OF    THE    IROX-BEARING   FORMATION    PROPER. 

The  iron-bearing  formation  proper  consists  of  the  various  colored 
cherts,  griinerite-magnetite-schists,  hematite,  magnetite,  and  limonite.  With 
the  iron-bearing  formation  proper,  but  in  quantity  very  subordinate  to  the 
cherts,  jaspers,  and  iron  oxide,  there  occur  some  greenish  to  gray  slates,  and 
also  graphitic  slates.  Upon  close  examination  under  the  microscojDe  these 
do  not  show  evidence  of  their  clastic  character.  False  bedding  would  per- 
haps indicate  their  clastic  origin,  but  no  other  evidence  of  this  has  been 
found  other  than  their  similarity  to  the  slates  above  mentioned,  which  grade 
into  the  recog'nized  elastics. 

The  white  chert,  in  combination  with  various  minerals  which  color  it, 
forms  the  bases  of  the  colored  varieties  of  cherts  already  enumerated  on 
page  181.  When  pure  the  white  chert  consists  of  quartz  varying  in  size 
of  grain  from  that  which  is  minutely  crystalline  to  that  which  is  somewhat 
more  coarsely  crystalline.  The  grains  are  polygonal  and  generally  more 
or  less  roundish.  In  one  case,  in  a  relatively  coarse-grained  chert,  three 
quartz  grains  were  observed  which  showed  a  roundish  core  outlined  by 
a  film  of  iron  oxide,  and  beyond  the  iron  oxide  a  zone  of  clear  quartz. 
This  secondary  enlargement  might  possibly  be  taken  as  evidence  of  the 
clastic  origin  of  the  grains,  but  this  structure  is  not  sufficient  evidence 
of  such  origin,  and  even  if  it  were  considered  sufiicient  proof  for  the 
grains  it  would  not  be   sufficient  evidence  to  prove  this   origin  for  the 


186  THE  VERMILION  IRON-BEARING  DISTRICT. 

suiTOUiidiug  cherts.  A  more  or  less  perfect  false  bedding'  observed  in 
some  of  these  cherts  (PI.  V,  B)  might  also  be  considered  the  result  of 
water  motion.  But  this  would  not  prove  the  cherts  to  be  of  mechanical 
origin,  as  we  find  this  structure  in  sediments  of  organic  origin  also.  The 
fine  grains  of  quartz  constituting  the  chert  contain  scattered  through  them 
minute  crystals  and  specks  of  magnetite  and  hematite  and  areas  of  limonite. 
In  some  places  these  crystals  are  very  small — mere  dust  as  it  were ;  in  others 
they  are  of  considerable  size.  At  some  places  the  quartz  grains  will  con- 
tain very  few  of  these  dust  specks;  at  others  they  are  nearly  full  of  tliem 
and  appear  almost  opaque.  The  crystals  of  the  iron  oxides  may  occur  at 
any  and  all  places  in  these  quartz  grains,  from  the  center  to  the  periphery, 
and  when  the  crystals  are  large  they  not  infrequently  extend  from  one 
quartz  grain  to  another,  running  across  the  junction  of  the  grains.  In  some 
cases  these  polygonal  g-raius  are  outlined  very  distinctly  by  iron  oxide, 
occurring  either  as  a  mere  film  or  as  a  layer  of  considerable  thickness. 
Instances  were  observed  where  the  grains  of  quartz  in  the  chert  were 
heavily  impregnated  with  particles  of  iron  ore  on  the  periphery,  leaving  but 
a  small,  fairly  clear  center.  Other  instances  were  observed  where  there  was 
less  of  the  clear  central  quartz  present,  and,  in  fact,  there  seemed  to  be  all 
gradations  from  these  cases  up  to  those  in  which  there  was  an  opaque  mass 
of  ore  giving  but  an  occasional  indication  of  the  presence  of  quartz. 
These  facts  seem  to  show  that  the  ore  in  these  rocks  is  not  primary,  but 
is  a  secondary  product  which  has  been  accumulated  either  in  bands  or  in 
irregular  masses  as  the  result  of  the  replacement  of  silica  by  iron  oxide. 

There  is  one  variety  of  the  chert  which  is  interesting-,  for  it  seems  to 
give  a  clue  to  the  siliceous  rock  which  has  been  replaced  by  the  ore.  This 
variety  is  the  gi-eenish  carbonate-bearing  chert  to  which  reference  has 
already  been  made.  Under  the  microscope  such  cherts  show  up  as  finely 
granular  aggregates  of  silica  in  normal  rounded  polygonal  g-rains,  but  asso- 
ciated with  the  silica  grains  is  a  carbonate  which  occurs  in  rounded  rhombo- 
hedra.  The  rounding  of  these  grains  is  not  the  result  of  mechanical  action. 
A  study  of  the  slides  shows  the  alteration  of  the  carbonate  to  limonite.  A 
further  change,  resulting  from  dehydration,  would  produce  a  hematite- 
bearing  chert,  and  in  cases  where  the  oxidation  of  the  carbonate  took 
place  with  access  of  insufficient  amount  of  oxygen  there  would  be  produced 
magnetitic  chert.     In  rocks  containing  a  large  propoi'tion  of  carbonate  and 


SOUDAN  FORMATION.  187 

a  relatively  small  proportion  of  chert  we  might  thus  get  as  a  result  of  the 
alteration  of  the  carbonate  a  ferruginous  rock,  possibly  with  alternate  bands 
rich  and  poor  in  iron.  These  presumably  served  as  a  nucleus  from  which, 
by  replacement,  were  derived  the  more  ferruginous  bands  and  ore  deposits. 
Detailed  descriptions  of  these  processes  have  been  given  elsewhere  by  Van 
Hise,"  and  will  not  be  discussed  in  tliis  place.  The  presence  of  the  limonite, 
hematite,  and  magnetite  in  the  cherts  gives  us  the  varieties  of  the  feri'uginous 
chert,  as  it  is  commonly  called — the  red  jasper  and  the  black  or  lean, 
hungry  jasper,  respectively. 

Associated  with  these  cherts  and  ores  are  the  griinerite-magnetite 
rocks.  These  are  not  present  in  large  quantity.  In  places  we  find  a  green 
rock  consisting  essentially  of  chert  with  griinerite  and  but  few  crystals  of 
magnetite;  at  other  places  there  are  rocks  in  which  magnetite  is  the  essential 
constituent  with  but  little  griinerite;  and  all  gradations  between  exist. 
This  griinerite  is  very  nearly  of  the  composition  of  the  hydrated  ferrous 
silicate  which  is  so  abundant  in  and  forms  such  a  conspicuous  part  of  the 
iron-bearing  rocks  of  the  Mesabi  range.  This  material  has  been  described 
in  detail  in  the  monograph  on  this  range  by  C.  K.  Leith.''  Altered  forms 
of  this  same  material  occur  in  the  iron-bearing  Gunflint  formation  at  the 
eastern  end  of  the  Vermilion  district.  The  griinerite  may  very  well  have 
been  derived  from  this  material  by  a  simple  process  of  dehydration,  or  it 
may  have  been  produced  from  an  iron  carbonate  by  silicification,  as  it  was 
in  the  Marquette  district,  as  described  by  Van  Hise."  Indeed,  since,  as  is 
concluded  later,  on  page  191,  iron  carbonate  was  the  original  rock  of  the 
iron-bearing  formation,  it  is  presumed  that  the  griinerite  was  for  the  most 
part  formed  by  the  silicification  of  the  carbonate. 

In  those  rocks  in  which  the  griinerite  occurs  no  traces  have  been  found 
of  the  peculiar  oval  and  globxilar  structures  so  characteristic  of  the  Biwabik 
and  Gunflint  rocks.  The  absence  of  these  rounded  bodies  is  not,  however, 
conclusive  evidence  that  they  did  not  originally  exist.     These  rocks  have 

a  The  Penokee  iron-bearing  series  of  Michigan  and  AVisconsin,  by  C.  R.  Van  Hise;  Mon.  U.  S. 
Geol.  Survey  Vol.  XIX,  1892,  p.  283  et  seq.  The  Marquette  iron-bearing  series  of  Michigan,  by  C.  R. 
Van  Hise;  Mon.  IT.  S.  Geol.  Survey  Vol.  XXVIII,  1897,  p.  402. 

6 The  Mesabi  iron-bearing  district  of  Minnesota,  by  Charles  Kenneth  Leith:  Mon.  U.  S.  Geol. 
Survey  Vol.  XLIII,  1903,  p.  101  et  seq. 

'The  Marquette  iron-bearing  series  of  Michigan,  by  C.  R.  Van  Hise:  Mon.  U.  S.  Geol.  Survey 
Vol.  XXVIII,  1897,  p.  367. 


188  THE  VERMILION  IRON-BEARING  DISTRICT. 

been  extremely  altered,  and  with  the  recrystallization  of  the  elements  there 
may  have  taken  place  complete  destruction  of  the  original  structui'es,  with 
the  exception  of  banding,  which  is  still  evident,  and,  in  fact,  this  may  have 
been  further  emphasized  by  the  rearrangement  of  the  constituents. 

ORIGIX. 

The  banded  structure  of  the  iron-bearing  formation  is  exceedingly 
regular  throughout  the  district.  It  is  difficult  to  say  how  persistent  tlie 
individual  bands  are,  however,  for  the  outcrops  do  not  allow  them  to  be 
traced  over  very  long  distances.  Most  of  the  bands  seen  at  a  particular 
exposure  persist  entirely  across  the  exposed  surface.  A  few,  however, 
run  out  to  a  feather  edge  even  on  small  exposures,  and  thus  disappear. 
The  251'esumption  is  that  all  the  bands  feather  out  within  a  shorter  or 
longer  distance.  This  banding  is  so  well  marked  and  so  eminently 
chai'acteristic  that  from  its  presence  alone  one  is  fully  warranted  in  making 
the  statement  that  the  structure  of  the  formation  is  essentially  that  of  a 
sedimentary  rock.  Furthermore,  the  composition. and  textxire  of  the  rocks 
making  up  the  formation  are  such  that  we  can  assert  with  confidence  that 
none  of  the  members  are  of  igneous  origin.  It  has  alread}^  been  stated 
that  in  places  the  iron  formation  proper — that  is,  the  interbanded  iron 
oxides,  cherts,  and  jaspers — overlie  conformably  a  series  of  clastic  rocks, 
beginning  at  the  bottom  Avitli  a  conglomerate  and  grading  upward  into 
finer  material,  and  also  that  we  find  similar  fine  clastic  material  interbanded 
with  the  iron  oxides,  cherts,  and  jaspers.  This  clearly  indicates  that  the 
rocks  of  the  iron  formation  are  of  sedimentary  origin.  The  presumption 
is  that  the  elastics  were  first  formed;  that  there  was  then  a  period  with 
changing  conditions,  during  which  tlie  slates  and  iron-formation  rocks  were 
interbedded,  and  that  finally  the  conditions  controlling  the  deposition  of 
rocks  of  the  iron-bearing  formation  became  more  persistent  when  the  orig- 
inal rocks  of  the  iron-bearing  formation  were  deposited.  This  enables  us 
to  explain  certain  characteristics  of  the  contact  between  the  green- 
stone and  the  iron  formation  which,  in  view  of  the  known  relations 
of  these  rocks,  could  not  otherwise  be  explained.  At  a  point  2,000  ^jaces 
east,  700  j^aces  south  of  the  southeast  corner  of  sec.  17,  T.  (i2  N.,  R.  13 
W.,  there  is  an  exposure  of  jasper  on  the  south  side  of  massive  green- 
stone.    At  this  exposure  there  seeins  to  be  a  gradation  of  the  greenstone 


SOUDAN  FORMATION.  189 

into  file  jasper.  On  the  nortli  side  of  the  exposm-e  the  greenstone  is 
massive,  bnt  as  it  nears  the  jasper  it  becomes  more  and  more  schistose,  and 
with  this  increasing-  schistosity  there  is  a  more  or  less  imperfect  banding, 
brought  about  b}'  an  alternation  of  bands  of  the  greenstone  discolored  with 
iron  with  those  which  are  not  so  colored.  Farther  south  the  true  jasper 
appears.  Here  apparently  is  a  fine-grained  mechanical  sediment  derived 
from  and  immediately  overlying'  the  greenstone,  a  deposit  which  is 
essentiallv  indistinguishable  from  the  greenstone  so  far  as  its  composition  is 
concerned,  its  mode  of  origin  being  indicated  by  the  imperfect  banding'  and 
the  presence  of  feiTuginous  material.  Here  there  is  no  well-marked  clastic 
deposit  between  the  greenstone  and  the  iron  formation,  and  this  is  one  of 
the  localities  where  the  quiet  conditions  controlling  the  deposition  of  the 
iron  formation  had  already  set  in,  while  in  other  parts  of  the  district 
elastics  were  being  formed.  For  instance,  in  sec.  10,  T.  63  N.,  R.  10  W., 
west  of  North  Twin  Lakes,  bands  of  the  jasper  lie  parallel  to  the  strike  of 
the  edge  of  the  greenstone  in  contact  with  it,  as  though  the  banded 
rocks  were  a  sedimentary  deposit  laid  down  upon  it  as  a  base.  But  no 
clastic  sediments  were  found  in  this  case  between  the  jasper  and  the  green- 
stone, their  absence  being  due  probably  to  the  fact  that  here,  likewise, 
the  conditions  were  those  of  quiet  deposition,  during  which  no  elastics 
could  be  formed.  From  the  fact  that  the  rocks  of  the  iron  formation  are 
conformable  with  the  clastic  sediments  near  their  contact  one  might  be 
inclined  to  infer  that  the  iron-formation  rocks  also  are  of  mechanical 
origin.  But  microscopic  examination,  however,  shows  that  the  cherts  and 
jaspers  and  ores  do  not  possess  the  minute  textvires  indicative  of  mechanical 
sediments,  although  one  case  has  been  noted  in  which  three  possibly  clastic 
quartz  grains  were  associated  with  the  chert,  thus  showing'  that  the  condi- 
tions fluctuated  from  those  suitable  for  the  formation  of  clastic  sediments 
to  those  giving  rise  to  organic  sediments,  as  shown  also  by  the  microscopic 
occurrence  of  interbanded  elastics  with  the  cherts. 

In  recapitulation  it  may  be  said  that  the  banding'  possessed  by  the 
iron  formation  and  the  slat}"  bands  associated  with  it,  which  show  true 
sedimentary  characters,  and  which  were  evidently  originally  of  detrital 
mud,  give  the  clearest  proof  of  the  sedimentary  origin  of  the  iron  formation 
itself.  The  question  which  may  next  be  raised  concerning  its  mode  of 
origin  is  whether  it  could  not  have  been  a  chemical  deposit.    N.  H.  Winchell 


190  THE  VEKMILION  IRON-BEARING  DISTRICT. 

and  H.  V.  Wiuchell,  who  have  had  excellent  oppoi-tunities  for  studying 
the  Minnesota  deposits,  have  so  construed  them."  A  condition  which 
would  admit  of  the  precipitation  from  a  sea  of  a  rock  as  acid  as  a 
chert,  consisting  esseutiall}^  of  pure  silica,  followed  immediately  by  the 
precipitation  of  rock  as  basic  as  the  bands  of  pure  iron  ore,  is  so 
anomalous  as  not  to  be  tenable.  The  only  explanation  which  seems  most 
nearly  to  meet  and  to  answer  the  requirements  of  texture,  structure, 
and  composition  is  that  the  rocks  are  now  in  a  very  different  condition, 
both  chemically  and  physically,  from  what  they  were  when  originally 
deposited.  Their  present  condition  may  be  interpreted  as  due  to  secondary 
changes  acting  upon  rocks  of  banded  character. 

The  exact  character  of  these  original  rocks  is  a  question  of  much 
moment.  As  the  source  of  the  iron-bearing  rocks  of  the  Mesabi  range 
Spurr  has  suggested  an  original  glauconitic  greensand,  partly  of  foraminif- 
eral  origin.  The  green  material,  called  by  Spurr  glauconite,  has  been 
carefully  studied  by  C.  K.  Leith,  and  has  been  found  to  be  not  glauconite 
but  a  hydrous  ferrous  silicate  without  any  potassium,  and  it  has  been  called 
"greenalite.'"*  Microscopic  study  of  the  rocks  from  the  Archean  iron- 
bearing  formation  of  the  Vermilion  district  has  shown  no  evidence  of  the 
former  existence  of  such  foraminiferal  rocks  or  glauconitic  greensands;  nor, 
indeed,  has  any  rock  been  found  Avhich  can  be  proved  to  be  the  original 
rock  from  whicli  the  ores  and  associated  rocks  have  been  produced.  How- 
ever, that  kind  of  rock  which  approaches  nearest  to  the  supposed  original 
rock  is  the  cherty  iron  carbonate  forming  a  part  of  the  iron  formation. 
Thi«  now  appears  to  represent  a  stage  in  the  process  of  metamorphism 
between  the  cherts  and  jaspers  and  ores  on  the  one  hand,  and  the  relatively 
pure  iron  carbonate  on  the  other. 

Tlie  presence  of  this  cherty  iron  carbonate  in  the  Vermilion  district,  in 
association  with  the  other  members  of  the  iron-bearing  formation,  oifers  also 
a  striking  analogy  between  this  district  and  those  on  the  south  shore  of  Lake 
Superior."  In  the  various  monographs  u^son  the  iron  ranges  in  the  United 
States  portion  of  the  Lake  Superior  region.  Professor  Van  Hise  has  presented 


"Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV,  1899,  p.  547;  Geol.  and  Nat. 
Hist.  Survey  of  Minnesota,  Bull.  No.  6,  1891,  pp.  105-111. 

'  ''The  Mesabi  iron-bearing  district  of  Minnesota,    by   C.  K.  Leith:  Mon.  U.  S.  Geol.   Survey 
Vol.  XLIII,  1903,  p.  115. 

''Tenth  Ann.  Rept.  II.  Si  Geol.  Survey,  pp.  896-.'W7;  Monofrraphs  U.  S.  Geol.  Survey  Vols.  XIX, 
XXVHI,  and  XXXVI. 


SOUDAN  FOEMATION.  191 

the  proof  in  favor  of  his  view  that  the  cherty  iron  carbonate  is  the  original 
rock  of  the  iron-bearing  formations.  In  the  Verraihon  district  tlie  iron 
carbonate  is  present  in  very  small  quantity,  but  it  is  significant  that  we 
find  siderite  in  large  quantity  in  the  ranges  east  of  this  but  in  line  of  its 
strike,  for  instance  near  Port  Arthur  and  in  the  Michipicoten  district. 

No  proof  of  the  supposition  that  a  cherty  iron  carbonate  is  the  original 
rock  of  the  Soudan  formation  has  been  found  in  the  Vermilion  district. 
From  the  analogy  of  this  with  the  other  iron-beariug  districts  of  the 
region  it  seems  most  probable  that  in  this,  as  in  the  districts  above  referred 
to,  the  cherty  iron  carbonate  was  the  chief  original  rock  of  the  iron-bearing 
i'ormation.  In  the  monographs  cited  above,  details  are  given  which  will 
enable  the  reader  to  follow  the  various  changes  which,  by  leaching  and 
deposition,  transform  the  cherty  iron  carbonate  into  chert,  jasper,  and  ore. 

While  stress  has  been  laid  upon  the  formation  of  the  jasper  as  the 
result  of  secondary  processes  acting  upon  an  originally  ferruginous  rock,  it 
must  be  stated  that  in  certain  places  in  the  district  the  jasper  doubtless 
owes  its  origin  to  processes  of  secondary  infiltration.  Such  jasper  occurs 
in  vein-like  form  and  in  irregular  bunches.  It  may  occur  also  as  the 
cement  of  the  brecciated  greenstones,  and  is  found  occasionally  lying 
between  the  greenstone  ellipsoids.  The  possibility  that  some  of  this 
jasper  may  be  due  to  secondary  action  must  of  course  be  admitted. 
This  is  admitted,  however,  only  for  very  small  masses.  These  masses 
were  jjrobably  formed  by  infiltration  in  openings  in  the  greenstones,  from 
above,  during  the  time  that  the  overlying  clierty  iron  carbonates  were 
being  changed  to  the  present  condition  of  the  iron-bearing  formation.  That 
the  infiltration  is  of  relatively  recent  date  in  one  particular  instance  is 
shown  by  the  presence  of  chert  veins  in  an  acid  porphyry  which  cuts  the 
jasper  and  the  greenstone  north  of  Mud  Creek  Bay.  Had  iron  been  present 
in  this  siliceous  solution  jasper  with  small  masses  of  ore  instead  of  the 
veins  of  chert  might  very  well  have  been  formed. 

REIiATIOlSrS  OF  SOUDAK  FORMATIO?^  TO  ADJACENT  FORMATIONS. 

RELATIONS  TO   THE  ELY  GREENSTONES. 

From  a  scientific  point  of  view  one  of  the  most  interesting  problems 
confronted  in  the  study  of  the  Vermilion  district  is  that  of  the  relation  of  the 
iron  formation  to  the  Ely  greenstones.  From  an  economic  standpoint  this 
is  also  one  of  the  most  important  problems.     It  was  likewise  most  puzzling 


192  THE  VER:\I1LI0N  IRON-BEARING  DISTRICT. 

and  most  difficult.  It  is  believed  that  the  relations  between  the  gi-eenstones 
and  iron-bearing  rocks  have  now  been  determined. 

In  approaching"  the  question  we  must  bear  in  mind  the  fact  that  this 
iron- bearing  formation  was  probably  at  one  time  spread  over  nearly  the 
entire  district  with  more  or  less  uniformity.  Ever  since  that  time,  however, 
it  has  been  subjected  to  mountain-making-  forces  and  active  processes  of 
erosion.  The  few  areas  of  this  formation  which  we  now  find  are  t>nly  those 
remnants  of  it  which,  being  infolded  in  the  underlying  formation  and 
-thereby  to  a  certaiii  extent  protected  from  erosion,  have  foi-tuuately 
remained. 

It  has  already  been  stated  that  most  commonly  the  contacts  between 
the  ffreenstone  and  the  iron  formation  are  wanting-.  Still  a  sufficient 
number  of  good  contacts  were  found  to  leave  no  doubt  whatever  as  to  the 
usual  relation.  Where  not  wanting  the  contact  is  usually  exposed  only 
over  a  very  small  area;  where  favorable  exposures  have  been  found  the 
usual  relations  are  such  as  are  described  in  the  following  jmragraphs. 

The  jasper  of  the  iron  formation  occurs  in  the  greenstone  in  lenses  of 
varying  size,  ranging  from  6  inches  in  width  upward,  the  smaller  ones 
being  very  common.  The  larger  ones  are  rarely  exposed  over  their  entire 
surfaces,  3^id  such  partial  exposures  make  it  very  difficult  to  trace  the 
various  bands  through  the  full  length  of  the  lenses,  and  it  is  practically 
impossible  to  recog-nize  the  same  bands  at  different  places  unless  one  can 
trace  them  over  the  intervening-  areas.  About  a  quarter  of  a  mile  north  of 
the  north  shore  of  Fall  Lake,  in  the  NE.  i  of  sec.  13,  T.  63  N.,  R.  12  W., 
the  jasper  is  found  in  the  greenstone  in  narrow  bands,  from  4  to  5  inches 
wide,  which  are  bent  sharply  upon  themselves,  showing  the  extreme  fold- 
ing to  which  they  have  been  subjected.  Similar  bands  varying  in  width 
from  a  few  inches  to  2  feet  are  found  in  the  NE.  \  of  sec.  7,  T.  63  N.,  R. 
11  W.,  infolded  in  the  amygdaloidal  ellipsoidal  greenstones,  and  there  form 
small  synclines  pitching  east.  South  of  Moose  Lake,  in  sec.  4,  T.  63  N., 
R.  9  W.,  on  the  narrow  neck  of  land  separating  the  two  small  lakes,  the 
infolding  of  the  jasper  in  the  gi-eenstone  is  well  shown.  In  many  excellent 
exposures  it  is  clear  that  the  jasper  overlies  the  greenstone.  One  especially 
good  exposure  showing  this  relation  very  clearly  can  be  seen  on  the  north 
side  of  Jas2)er  Lake.  Here  the  iron-bearing  formation  is  in  Canadian  ter- 
ritory, but  is  in  direct  continuation  of  and  really  in  couuiectiou  with  the 


SOUDAN  FORMATION.  19 


Q 


Vermilion  range  of  Minnesota.  At  this  place  the  jasper  stands  with  a  clip 
of  about  85°  west,  with  the  greenstone  both  to  the  east  and  to  the  west  of 
it.  One  might  be  inclined  to  say,  and  with  good  reason  from  this  part  of  the 
exposure,  that  the  jasper  is  included  in  or  interbauded  with  the  greenstone. 
However,  as  we  go  over  the  end  of  the  exposure,  coming  south  down  the 
hill  along  the  strike,  it  can  be  seen  that  this  relation  is  due  to  the  exceed- 
ingly close  infolding.  The  jasper  is  closely  compressed  and  lies  in  a 
syncline  of  greenstone,  and  in  the  inclined  section  through  the  jasper  and 
greenstone  displayed  on  the  hill  slope,  the  greenstone  can  be  seen  to  pass 
down  under  the  jasper,  from  both  the  east  and  the  west  sides.  Indeed, 
at  one  place  a  subordinate  anticline  of  greenstone  was  observed  projecting 
up  through  the  jasper,  the  jasper  wrapping  around  it  and  dipping  away 
from  it.  On  Otter  Track  Lake  essentially.  ,th,e  same  relations  can  be  seen  to 
exist.  Actual  contact  between  the  irqufbearing  formation  and  the  green- 
stone was  observed,  but  no  fragmental  material  occurred  between  them. 
Clearly,  however,  the  jasper  overlies  the  g-reenstone,  having  been  deposited 
upon  this  as  a  basement,  for  the  bands  run  parallel  with  the  contours  of  the 
greenstone  mass.  In  this  particular  instance  the  exposures  are  riot  quite  so 
good  as  those  upon  Jasper  Lake,  but  are  still  good  enoug-h  to  enable  one  to 
determine  the  relations  with  certaint}^.  On  Emerald  and  Big'  Rock  lakes, 
both  of  which  lie  in  Canadian  territory,  north  of  Knife  Lake,  there  are 
several  exposures  of  jasper  in  intimate  association  with  the  greenstone. 
All  the  jaspers  of  Emerald  Lake,  especially,  are  beautifully  and  curiously 
folded.  This  folding  is  particularly  well  seen  on  the  western  edge  of  the 
large  island  about  a  mile  and  a  half  west  of  the  end  of  the  lake.  As  one 
rows  along  the  shore  he  may  see  most  intricately  folded  jasper  bands  which 
closely  resemble  the  jaspers  on  Otter  Track  Lake  figured  by  H.  V.  Winchell 
in  the  Minnesota  reports.  There  are  fan-shaped  folds  and  curious  inter- 
loekiugs  which  would  be  almost  incredible  if  not  seen.  The  jasper, 
although  so  rigid,  has  evidently  obeyed  the  law  of  flowage  by  filling  up 
every  chink  and  corner  throughout  the  mass.'  It  is  still  somewhat  question- 
able how  far  this  formation  may  have  been  folded  before  it  was  jasperized. 
At  this  ledge  upon  the  island  the  broad  jasper  bands  were  peen  to  be  bent 
around  in  a  curve  having  a  radius  of  from  2  to  4  inches,  sections  across 
them  giving  a  roundish  surface,  making'  them  appear  almost  like  a  series  of 
closely  laid  pipes. 

MON  XLV-03 13 


194  THE  VERMILION  IRON-BEARING  DISTRICT. 

In  the  cases  mentioued  the  iron-bearing-  formation  was  found  in  imme- 
diate contact  with  the  greenstone  at  a  great  number  of  places.  In  all  cases 
the  contact  is  sharp.  There  is  no  gradation  from  the  greenstone  through  a 
distinctly  clastic  transitional  rock  into  the  iron-bearing  formation.  The 
field  relations  described  are  evidently  such  as  could  be  produced  only  by 
the  closest  infolding  of  a  superimposed  rock  in  the  rock  below  it.  This 
intricate  infolding  is  furthermore  shown  by  the  contorted  chai'acter  of  the 
banded  iron  formation. 

The  iroii  formation  is  not  confined  to  the  normal  relatively  unmetaraor- 
phosed  greenstones,  but  is  also  found  in  those  which  have  been  metamor- 
phosed by  contact  with  the  granite  into  the  amphibole-  and  mica-gneisses, 
as  in  the  occurrence  in  the  SE.  \  of  sec.  32,  T.  62  N.,  R.  13  W.,  south  of 
Burntside  Lake.  In  passing,  it  may  be  mentioned  that  the  presence 
of  the  iron  formation  in  these  amphibole-schists  and  gneisses  is  further 
proof  of  the  correctness  of  the  statement  already  made  that  these  are  but 
much  metamorphosed  greenstones,  and  essentially  the  same,  at  least  so 
far  as  their  original  condition  is  concerned,  as  the  greenstones  which  are 
not  very  far  distant. 

Reference  has  been  made  to  the  fact  that  the  jasper  at  the  contact 
with  the  greenstone  is  sometimes  brecciated.  One  especially  clear  case  of 
this  is  the  occurrence  on  the  south  side  of  the  North  Lee  pit.  Here  the 
jasper  fragments  derived  from  the  iron-bearing  formation  are,  especially 
near  the  greenstone,  more  or  less  completely  surrounded  by  a  matrix 
of  the  adjacent  greenstone,  which  evidently,  under  the  pressure  exerted 
upon  it,  became  more  readily  plastic  than  did  the  more  brittle  jasper.  Com- 
monly brecciation  does  not  occur  at  the  contact.  The  greenstone  is  usually 
schistose  along  such  contacts.  This  is  illustrated  220  paces  north  of  the 
southeast  corner  of  sec.  15,  T.  62  N.,  R.  13  W.  Here  there  is  a  bare  roche 
moutonnee,  which  consists  for  the  most  pai't  of  the  very  dense  massive 
greenstone.  At  one  place  there  was  found  a  belt  of  schistose  greenstone 
about  3  feet  wide  lying  immediately  next  to  the  massive  form  above 
mentioned.  In  this  schistose  greenstone  there  are  two  naiTOw  bands  of 
east-west  striking  jasper,  each  about  4  inches  in  width.  These  were 
separated  from  each  other  and  from  the  massive  greenstone  by  tlie  schistose 
form  of  the  greenstone,  whicli  to  the  south  as  well  as  to  the  north  grades 
into  the  massive  variety.     It  is  hardly  possilile  to  interpret  this  oci'iirrence  as 


SOUDAN  FORMATION.  195 

anytliino'  else  than  a  case  of  infolding.  As  the  result  of  the  accompanying 
movement,  the  gi'eenstone  was  rendered  schistose  immediately  adjacent  to 
the  jasper  and  nearest  to  the  plane  between  the  rocks  of  diverse  character 
along  which  the  greatest  movement  naturally  took  place,  this  schistosity 
diminishing  in  degree  away  from  the  plane  of  greatest  movement. 

In  several  places  in  the  district  careful  search  made  expressly  therefor 
has  disclosed  the  greenstone  with  a  conglomerate  lying  on  it  and  con- 
sisting of  material  derived  from  the  greenstone.  The  conglomerate  is 
followed  by  finer-grained  clastic  sediments  of  essentially  the  same  character 
as  the  conglomerate  itself,  and  these  sediments  are  in  turn  succeeded  by 
the  iron  formation  proper,  consisting  of  the  normal  cherts,  jasper,  and  iron 
oxide  in  alternate  bands.  Occasionally  a  clastic  band  occurs  with  the  iron 
formation  jjroper.  Such  definite  relationships  were  observed  at  several 
places,  as  in  sec.  10,  north  of  Armstrong  Lake.  Here  the  greenstone  is 
overlain  by  a  conglomerate  derived  from  it,  which  is  succeeded  to  the  north 
by  the  iron  formation,  which  contains  a  large  quantity  of  iron  pyrites.  The 
greenstone  also  contains  larg-e  quantities  of  the  iron  pyrites  scattered 
through  it  in  crystals  which  liave  to  a  great  extent  been  changed  to 
limonite.  Another  locality  is  north  of  Robinsons  Lake,  just  south  of  the 
north  quarter  post  of  sec  7,  T.  62  N.,  R.  13  W.  A  number  of  other  places 
were  found  in  which,  however,  the  relations  were  not  quite  so  clear,  the 
complete  sequence  being  interrupted  by  lack  of  exposures ;  they  are  there- 
fore not  referred  to  specifically.  The  above-mentioned  relations  show 
clearly  that  the  main  part  of  the  iron  formation  rests  upon  the  greenstone 
as  a  basement,  and  consequently  is  younger  than  the  greenstone. 

But  is  all  of  the  iron  formation  younger  than  all  of  the  greenstones  of 
the  Vermilion  area'?  Apparentl}^  not,  but  there  is  occasionally  an  inter- 
bedding  of  the  iron  fonnation  with  some  of  the  greenstone.  The  evidence 
for  this  is  found  in  the  distribution  of  the  iron  formation  in  long  belts 
separated  from  one  another  by  areas  of  varying  width  underlain  by  the 
greenstones.  These  belts  have  been  traced  for  various  distances,  in  one 
case  for  a  distance  of  16  miles.  In  no  case  is  the  iron-bearing  formation 
continuously  exposed  over  such  extent,  but  the  exposures  are,  nevertheless, 
so  numerous  as  to  show  conclusively  that  the  intervening  areas  without 
exposures  are  underlain  by  the  iron-bearing  rocks.  Associated  with  the 
iron-formation  rocks  of  these  belts  there  is  more  or  less  greenstone.     This 


196  THE  VERMILION  IKON-BEARING  DISTRICT. 

is  frequently  found  in  contact  with  the  iron-bearing  rocks  and  tends  to 
separate  the  formation  into  a  number  of  small  belts  whose  continuity  can 
not  be  traced  on  account  of  the  rarity  of  exposures.  The  greenstones 
which  occur  in  this  iron-foriuation  belt  are  of  different  kinds,  and  it  is 
supposed  that  they  inay  re^^resent  flows  of  greenstone  geologically  con- 
temporaneous with  rocks  of  the  iron  formation.  The  exposm-es  are  so 
isolated,  however,  that  it  is  not  feasible  to  coiinect  them  and  separate  the 
rocks  into ■  individual  flows  or  sills.  Furthermore,  it  is  in  these  belts  of  the 
iron-beai'ing  rocks  that  the  above-mentioned  clastic  rocks  derived  from  the 
greenstones  and  underlying  conformably  the  iron-bearing  formation  have 
been  found.  In  view  of  these  facts,  we  are  led  to  believe  that  some  of  the 
rocks  of  the  iron-bearing  formation,  while  resting  upon  the  basement  of 
greenstones,  are  likewise  overlain  by  greenstones,  or,  in  other  words,  that 
some  of  the  iron  fonnation  is  interbedded  with  greenstones  which  are  of 
volcanic  origin. 

Thus  the  clastic  sedimentary  deposits  derived  from  and  overlying  the 
lava  flows  may  grade  up  into  nonclastic  sediments.  The  conditions  of 
sedimentation  var}'  from  place  to  place  in  the  area,  hence  we  get  a  gradual 
change  from  mechanical  sediments  to  organic  sediments  (cherty  ferruginous 
carbonates).  Where  the  conditions  were  not  favorable  for  the  formation  of 
the  clastic  sediments  the  nonclastic  sediments  were  deposited  without  the 
conglomerates  and  graj^wackes  intervening  between  them  and  their  igneous 
rock  basement.  Hence  we  no^^'  find  them  resting  upon  the  greenstones  with 
a  sharp  line  of  demarcation  between  them,  or  at  most  with  a  narrow  zone 
of  schistose  gTeenstone  intervening.  These  sediments  were  in  their  turn  in 
some  cases  buried  by  lava  flows,  which  again  at  a  later  date  were  covered 
by  succeeding-  sedimentary  deposits.  These  processes  continued  through- 
out a  shorter  or  longer  period.  It  is  due  to  this  fact  that  such  intimate 
relationship  exists  between  the  greenstones  and  the  associated — in  the  main 
youngei- — iron-bearing  formation.  As  a  result  of  this  intimate  relationship 
it  has  been  impossible  to  logically  separate  the  two  in  a  more  marked  way 
than  has  been  done  in  the  above  pages.  AVhile  of  a  distinct  method  of 
origin,  their  formation  took  place  within  essentially  the  same  period  of  time. 
In  general,  however,  it  is  possible  to  recognize  the  greenstone  as  the  true 
basement  rock  of  the  district,  correlative  with  the  Archean  rocks  of  the 
other  Lake  Superior  iron-bearing  districts. 


SOUDAN  FOKMATION.  197 

^  Within  the  greenstone  are  areas  of  jasper  of  very  irregular  shape  and 
size  which  do  not  possess  the  regular  banding  seen  in  the  jasper  of  the  large 
areas  of  the  iron  formation.  Such  small  areas  of  jasper  are  not  uncommon 
in  the  northern  half  of  sec.  21,  T.  62  N.,  R.  14  W.,  from  1,150  to  1,260 
paces  north,  950  paces  west  of  the  southeast  corner;  again,  in  sees.  1,  2, 
and  3,  T.  61  N.,  R.  15  W.  These  jasper  areas  are  of  irregular  shape  and 
appear  to  owe  their  origin  to  a  process  of  infiltration  similar  to  that  which 
forms  veins.  This  is  clearly  the  mode  of  origin  of  these  irregular  masses  of 
jasper  which  occur  in  the  midst  of  the  ellipsoidal  greenstones,  filling  the 
angular  interstices  between  the  ellipsoids  (see  page  139).  Some  of  these 
irregular  masses  may  be  remnants  of  the  iron-bearing  formation  deposited 
in  irregularities  of  the  underl^dng  rocks,  but  this  mode  of  origin  could  not 
be  proved  for  any  occurrence.  There  is  a  further  possibility  that  some  of 
these  masses  may  be  inclusions  of  the  iron  formation  in  the  eruptive  green- 
stones. This  explanation  has  been  offered  by  H.  V.  Winchell  and  others 
for  a  large  part  of  the  iron  formation  of  this  district  and  has  been  cited  as 
proof  that  the  iron  formation  was  older  than  the  greenstones.  However,  as 
already  shown,  the  presence  of  the  conglomerates  clearly  disproves  this  age 
relationship  for  the  greater  part  at  least  of  the  iron  formation.  Infolding 
may  as  readily  explain  the  intimate  character  of  the  relationship  between 
the  greenstone  and  the  iron  formation  as  the  suggested  intrusive  relationship. 
For  instance,  when  we  find  small  isolated  lenses  of  the  iron  formation  lying 
in  the  greeiistone,  with  the  surface  only  exposed,  or  when,  as  not  uncom- 
monly happens,  narrow  bands  of  the  formation  are  bounded  on  two  sides 
by  the  greenstone,  their  lateral  extension  being  concealed  in  the  other  two 
directions,  the  exposures  are  too  imperfect  to  enable  one  to  determine  the 
exact  relation  of  each  mass  of  the  iron-bearing  formation  to  the  greenstone. 
Nevertheless,  when  considered  in  connection  with  other  instances,  such  as 
have  been  mentioned  and  described,  where  the  relations  of  the  large  masses 
of  iron  formation  to  the  greenstone  are  clearly  those  due  to  infolding,  it 
will  be  readily  admitted  that  the  relations  of  the  other  doubtful  cases  are 
also  best  explained  as  due  to  this  same  thing.  Especially  are  we  inclined 
to  this  conclusion  when  the  close  folding  to  which  the  rocks  of  the  district 
have  been  subjected  is  fully  recognized.  Admittedly  some  of  the  greenstones 
may  be  younger  than  some  of  the  iron  formation;  for  instance,  the  sills 
and  dikes  forced  into  the  iron-bearing  belts  toward  the  close  of  the  period 


198  THE  VERMILION  IRON-BEARING  DISTRICT. 

of  volcanic  activity,  or  some  old  flows  which  have  poured  out  from  the  land 
into  the  adjacent  sea  while  the  iron-bearing  formation  was  being  dej^osited. 
In  no  case,  however,  has  a  jasper  mass  been  found  which  could  be  conclu- 
sively shown  to  be  included  in  the  greenstone  as  a  result  of  igneous  intrusion. 

EESUMi:  OF  RELATIONS  TO  ELY  GREENSTONES. 

In  view  of  the  above-described  modes  of  occurrence  of  the  iron 
formation  in  association  with  the  greenstone,  we  reach  the  following 
conclusions  concerning  the  age  relations  of  the  two :  A  portion  of  the  iron 
formation — and  this  appears  to  form  by  far  the  predominant  part  of  it — is 
clearlv  younger  than  the  greenstone;  for  instance,  those  masses  of  the 
formation  that  overlie  the  clastic  sediments  which  have  plainly  been  derived 
from  the  ixnderljdng  greenstone.  Other  very  subordinate  portions  of  the 
formation  are  interbedded  with  the  greenstone,  and  hence  are  partly 
contemporaneous  with  it.  This  is  shown  in  those  cases  where  there  is 
greenstone  overlying  the  iron  formation.  Moreover,  it  is  not  improbable, 
although  not  susceptible  of  definite  proof,  that  some  of  the  smaller  areas  of 
the  iron  formation  are  included  in  a  greenstone  which  has  been  later 
intruded  through  the  iron  formation.  Lastl}',  small  areas  of  jasper,  chert, 
and  ore,  similar  in  general  characters  to  the  iron-bearing  rocks,  are  second- 
arv  infiltration  products. 

In  order  to  get  a  clear  understanding  of  the  conditions  which  would 
permit  such  a  variety  of  relationships  between  the  greenstones  and  the  iron 
formation,  it  is  necessary  that  we  call  to  mind  the  conditions  under  which 
these  two  formations  originated.  A  study  of  the  greenstones  has  led  to 
the  conclusion  that  they  were  formed  by  volcanic  outbursts.  Just  as  at 
the  present  day  we  have  lavas  and  tuff  masses  outpoured  upon  the  land 
and  partly  occupying  adjacent  water  areas,  sedimentary^  deposits  being 
formed  off"shore  where  the  conditions  are  favorable  for  them,  just  so  did 
we  liave  similar  conditions  in  the  early  history  of  the  Vermilion  district. 
As  a  consequence  of  this  volcanic  outburst  on  the  land  and  the  simultaneous 
formation  of  sedimentarj^  deposits  in  the  sea,  we  now  find  the  two 
intermingled.  In  this  way  we  can  conceive  that  clastic  sedimentarj- 
deposits  might  be  derived  from  aud  overlie  lava  flows,  and  grade  up  into 
nonclastic    sediments.     Where    conditions    were    not    favorable    for   the 


SOUDAN  FORMATION.  199 

formation  of  clastic  sediments,  nonclastic  sediments  were  formed  without 
the  conglomerates,  etc.,  intervening  between  them  and  their  igneous-rock 
basement;  hence  we  now  find  them  i-esting  upon  the  greenstones  with  a 
sharp  line  of  demarcation  between  them.  Conditions  of  sedimentation 
varied;  hence  we  get  a  gradual  change  from  mechanical  sediments  to 
organic  sediments  (iron-bearing  rocks).  The  sediments  in  their  turn  were 
buried  by  lava  flows,  which  again  at  a  later  date  were  covered  up  by 
succeeding  sedimentary  deposits,  and  so  on.  So  far  as  we  can  ascertain, 
volcanic  activity  continued  only  during  the  time  when  the  lowest  sediments 
were  being  formed.  We  have  no  evidence  that  volcanic  actiAdtv  continued 
during  that  later  period  in  which  the  iron-bearing  sediments  were  deposited. 
Lastly,  through  this  series  of  sediments  and  lavas  intrusive  masses  Avould 
be  forced,  which  would,  in  some  places  at  least,  include  portions  of  the 
sedimentar}^  deposits  as  well  as  of  the  lava  associated  with  them. 

RELATIONS   TO    THE    ARCHEAN    ACID    INTRUSIVES. 

On  Soudan  Hill  and  elsewhere  the  iron-bearing  formation  is  intruded 
by  acid  intrusives  belonging  to  the  Archean  eruptive  series.  At  the 
Eaton  explorations  at  the  SE.  ^  of  sec.  7  and  SW.  J  of  sec.  8,  T.  62  X., 
R.  14  W.,  the  jasper  and  ore  are  cut  by  granite-porphyry,  which  carries  very 
large  quartz  phenocrysts.  Also  north  of  Mud  Creek  Bay,  in  the  SE.  J  of  sec. 
1,  T.  62  N.,  R.  15  W.,  the  jasper  is  both  cut  by  and  included  in  a  granitic 
eruptive.  On  the  north  shore  of  the  lake,  in  sec.  18,  T.  62  N.,  R.  12  W., 
the  jasper  is  cut  by  granite-porphyry.  Granite  dikes  cut  the  iron-formation 
belt  south  of  Ely  in  sees.  3  and  4,  T.  62  N.,  R.  12  W.  The  jasper  belt 
extending  through  the  south  half  of  sec.  10,  T.  63  N.,  R.  10  W.,  is  also  cut 
by  dikes  of  granite  and  granite-porphyry.  Similar  occurrences  coiild  be 
multiplied,  all  showing  the  iron-bearing  foi-mation  cut  by  the  younger  acid 
eruptives,  but  it  is  not  necessary  to  further  emphasize  this  relationship, 
which  is  indisputably  clear. 

RELATIONS  TO  OVERLYING  SEDIMENTS. 

The  iron  formation  is  in  places  overlain  b}^  a  series  of  sediments  of 
clastic  origin.  Where  contacts  between  these  series  were  observed  it 
was  found  that  the  relationship  existing  was  that  of  two  unconformable 
sedimentary   deposits.     The  proof  of  this  is  in  the   fact  that  the  upper. 


200  THE  VERMILION  IRON-BEARING  DISTRICT. 

3^ounger  sedimentary  series  contains  in  it  fragments  of  the  underlying, 
older  series,  the  iron  formation.  The  details  concerning  the  relations  of 
these  two  formations  are  given  under  the  discussion  of  the  later  sediments. 

RELATIONS   TO   BASIC   EROPTIVES. 

In  several  places  the  iron-bearing  formation  is  found  to  have  been 
cut  by  basic  eruptives.  Thus,  for  examj^le,  in  sec.  27,  T.  62  N.,  R.  15  W., 
the  jasper  is  cut  by  dikes  of  greenstone  which  must  be  younger  than  the 
jasper,  and  very  probably  belong  with  the  Lower  Hurouian  basic  intrusives. 
Again,  south  of  Ely,  in  sees.  3  and  4,  T.  62  N.,  R.  12  W.,  basic  intrusives 
are  found  to  cut  across  the  iron  formation. 

AGE. 

From  the  preceding  paragraphs  it  will  have  been  learned  that  the 
Soudan  formation  is  in  general  younger  than  the  Ely  greenstone,  the  oldest 
rock  of  the  district,  but  on  the  whole  so  intimately  associated  with  it  that 
the  two  must  be  considered  as  belonging  to  the  same  great  period  of  the 
earth's  history,  the  Archean. 

THICKNESS. 

It  is  impossible  to  make  any  reliable  estimate  of  the  thickness  of  the 
iron  formation,  and  this  for  many  reasons.  In  the  first  place,  the  exposures 
of  the  formation  are  so  isolated  and  the  formation  itself  throughout  is  of 
such  uniform  character  that  it  is  impossible  to  recognize  the  same  horizons 
in  it  in  different  parts  of  the  district.  No  definite  basement  has  been  found 
from  which  to  begin  an  estimate  of  the  thickness.  The  ver}-  close  folding' 
to  which  the  rocks  of  the  district  have  been  subjected  adds  to  the  complica- 
tions. The  thickness  of  the  Soudan  formation,  as  inferred  from  its  surficial 
extent  rather  than  from  any  definite  measurements,  is  presumed  to  reach 
several  hundred  feet 

INTERESTING  LOCALITIES. 

On  the  bare  hills  just  nortli  of  the  northernmost  houses  of  the  town  of 
Tower  there  are  a  number  of  exposures  that  show  the  relations  between 
the  iron  formation  and  the  associated  rocks.  For  instance,  the  southern- 
most exposures  on  these  hills  are  conglomerates  made  up  of  pel^bles  of 
jasper,  slate,  and  chert.     These  materials  have  been  derived  from  the  iron- 


SOUDAN  FOKMATION.  201 

formation  rocks  which  immediatel}'  underhe  them,  aud  in  phices  are  seen 
in  juxtaposition  with  the  conglomerate.  Erosion  has  removed  the  con- 
gk^merate  in  some  cases,  leaving  small  areas  of  the  jasper  only  a  few 
yards  in  extent  surrounded  by  the  conglomerate.  Associated  -with  the 
jasper  at  this  place  are  very  narrow  bands  of  black  graphitic  slate.  It 
is  from  some  of  these  bands  that  the  fragments  of  slate  in  the  overlying 
conglomerate  have  been  derived.  There  is  an  area  about  100  yards 
wide  on  the  slope  of  this  hill  in  which  the  conglomerate  and  underlying- 
jasper  are  intimately  associated.  Nox'th  of  these  exposures  occur  the 
iron-formation  rocks,  consisting  of  jasper,  cherts,  and  iron  ore  interbanded 
and  closely  infolded  with  the  green  schists.  The  iron  formation  has  its 
normal  characters,  which  have  already  been  described.  The  green 
schist  associated  with  it  ^Dossesses  an  exceedingly  well-developed  fissility, 
*  which  strikes  N.  70°  E.  The  schist  is  much  crumpled  in  places  and 
show-s  minor  faulting,  with  bending  of  the  lines  of  schistosity.  The 
faults  cut  across  the  schistosity  at  an  angle  of  45°  aud  extend  about 
northwest-southeast.  This  schist  is  impregnated  in  areas  of  irregular 
outline  with  u'on,  especially  along  the  southern  side  of  the  exposures 
nearest  the  jasper.  A  great  deal  of  vein  quartz  has  also  been  iniiltrated 
into  the  schist  and  is  found  in  thin  sheets  marking  the  planes  of  schistosity, 
aud  also  in  fine  systems  of  rectangular  veins  which  cut  the  schistosity. 
The  green  schist  and  iron  formation  are  most  intimately  infolded,  forming 
a  series  of  anticlines  and  synclines  having  very  steep  pitches.  The  axes  of 
the  folds  have  a  strike  approximately  coinciding  with  the  strike  of  the 
schistosity  in  the  green  schists,  N.  70°  E.,  showing  an  exceedingly  close 
folding  of  the  rocks.  A  great  number  of  these  small  folds  was  observed, 
and  in  some  cases  the  bands  of  iron  formation  or  of  schist  could  be  traced 
through  several  folds.  It  is  very  clear  that  the  close  infolding  here  has 
produced  a  kind  of  fluted  structure  which  is  best  developed  on  the  saddle 
connecting  Tower  and  Lee  hills,  which  stand  en  echelon  to  each  other  from 
northwest  to  southeast. 

The  same  kinds  of  intricate  plications  of  jasper  and  green  schist  can 
be  seen  on  the  numerous  exposures  on  Soudan  Hill.  An  especially  good 
one  occurs  on  the  north  flank  of  the  hill  about  1,040  paces  north,  330  paces 
west  of  the  southeast  corner  of  sec.  28,  T.  62  N.,  R.  15  W.  Numerous 
other  cases  may  be  observed  west  of  the  Montana  pit  and  on  the  north 


202  THE  VEKMII.ION  IRON-BEARING  DISTRICT. 

flank  of  the  hill  north  of  No.  8  shaft.  Several  good  exposures  occur  also 
along-  the  road  leading  from  the  top  of  the  hill  to  the  compressor,  and  also 
near  the  top  of  the  hill  on  the  right  side  of  the  tramway  leading  to  the 
compressor. 

On  the  south  shore  of  Vermilion  Lake  in  the  SE.  J  of  sec.  20,  T.  62 
N.,  R.  15  W.  on  the  point  between  Swede  Bay  and  Vermilion  Lake  there 
are  a  number  of  exposures  of  the  iron  formation  in  close  association 
with  the  later  sedimentaries.  Lidenting  the  northeast  shore  of  this  point 
there  is  a  small  bay,  on  the  west  shore  of  which  (almost  due  west  of  a 
small  reef  of  conglomerate)  there  is  an  exposure  of  jasper.  This  jasper 
exposure  is  only  about  75  paces  across  and  is  extremely  plicated,  showing 
a  number  of  distinct  anticlines  and  synclines  with  axes  striking  east  and 
west  and  plunging  east  at  the  high  angle  of  80°.  On  this  exposure  there 
are  also  some  beautiful  examples  of  friction  breccias  on  a  small  scale. 
Following  this  exposure  inland  we  pass  over  several  other  jasper  expo- 
sures showing  notliiug  of  especial  interest,  so  far  as  the  iron  formation  itself 
is  concerned,  but  lying  above  the  iron  formation  here  and  there  we  find  a 
patch  of  conglomerate  derived  from  it  and  showing  distinctly  its  relation- 
ship to  the  underlying  jaspers.  About  halfwaj-  across  the  point  we  reach 
a  place  where  a  considerable  area  of  the  jasper  is  exposed.  At  this  point 
the  jasper  is  extremely  plicated,  like  that  upon  the  shore.  The  axes  of  the 
plications  strike  about  N.  75°  E.  and  form  an  arc  of  an  oval  whose  long 
axis  trends  N.  75°  E.  It  seems  very  clear  that  we  are  at  this  place  just 
east  of  or  near  the  apex  of  a  small  east-west  anticline.  A  conglomerate 
lies  around  the  jasper,  bordering  it,  with  an  occasional  patch  still  remain- 
ing on  the  top,  and  hence  lying  in  the  midst  of  the  jasper  area.  The 
conglomerate  has  its  greatest  development  to  the  north,  while  to  the  south 
the  slates  are  most  common. 

Good  exposures  near  the  North  Lee  pit  on  Lee  Hill,  northeast  of  Tower, 
offer  excellent  opportunities  for  a  study  of  the  relations  between  the  green 
schist  and  the  jaspers.  A  green  chloritic  schist  is  exposed  in  a  practically 
solid  mass  for  about  125  paces  to  the  south,  theii  follows  the  jasper,  and 
again,  north  of  the  jasper,  comes  the  solid  greenstone  for  something  like  75 
paces,  where  an  alternation  of  jasper  and  green  schists  begins,  eventually 
followed,  still  farther  north,  by  a  lai'ge  mass  of  greenstone.  The  schistosity 
in  tlic  scliists  strikes  east  and  west.     Nearest  the  main  mass  of  the  jasper  the 


SOUDAN  FOKMATION.  203 

schist  contains  a  few  small  lenses  and  stringers  of  jasper  and  chert,  which 
extend  along  the  planes  of  schistosity  and  clearly  have  been  introduced  into 
the  rock  since  it  was  rendered  schistose.  The  surface  exposures  of  the  schist 
east  of  the  jasper  are  disconnected,  but  it  is  reported  by  the  mine  captain 
who  had  charge  of  the  North  Lee  pit,  now  filled  with  water,  that  under- 
ground the  jasper  is  cut  off  by  the  greenstone  which  surrounds  it  to  the 
east.  The  jasper  exposed,  especially  to  the  west  of  the  North  Lee  pit,  is 
very  badly  fractured,  and  ore  has  been  introduced  since  the  fracturing, 
healing  the  cracks  and  filling  the  cavities.  Moreover,  along  the  edge  of 
the  pit  from  which  the  ore  body  has  been  removed  there  can  be  seen 
remnants  of  banded  lean  ore.  These  bands  are  continuous  with  the  bands 
in  the  jasper  lying'  next  to  and  continuous  with  the  ore.  There  is  here  a 
gradation  from  a .  small  mass  of  rich  ore  through  a  very  hematitic  jasper 
into  the  normal  jasper.  The  fracturing  and  brecciation  of  the  jasper  are 
indicative  of  the  extreme  folding  to  which  it  has  been  subjected.  This 
is  further  shown  by  the  presence  of  a  fairly  extensive  breccia  along  the 
contact  between  the  southern  wall  of  green  schist  and  the  jasper.  Along- 
this  contact  has  occurred  brecciation  of  the  chert  and  jasper,  and  angular 
to  subangular  fragments  of  these  form  the  pebbles  for  the  most  part,  with 
the  green  schist  occurring  chiefly  as  the  matrix.  Occasionally  a  fragment 
of  the  green  schist  occurs,  also  as  a  pebble,  in. the  mati'ix.  Since  the 
formation  of  the  breccia,  iron  has  been  infiltrated  into  it,  and  this  has 
tended  in  many  places  to  cement  it  together,  so  that  at  some  localities  it 
contains  A^ery  nearly  enough  hematite  to  be  considered  a  lean  ore.  In 
places  along  this  contact  a  great  deal  of  white  quartz  has  been  infiltrated, 
showing  that  water  has  been  verj^  active  here. 

Li  the  SW.  4  of  the  NW.  i  of  sec.  3,  T.  61  N.,  R.  15  W.,  there  are 
a  number  of  exposures  of  jasper  which  show  extreme  plication  and 
infolding  of  jasper  and  green  schist.  Here  also  there  is  a  band  of  breccia 
consisting  of  fragments  of  jasper  and  chert  in  a  green  schist  matrix. 
Similar  breccias  have  been  observed  also  at  other  places  throug-hout  the 
district.  The  jasper  upon  these  exposures  is  for  the  most  part  that  which 
is  usually  called  the  black  or  hungry  jasper.  It  is  a  magnetitic  chert. 
This  in  places  shows  a  fine  banding,  probably  sedimentary  banding.  In 
the  NE.  4  of  the  SE.  4  of  sec.  1,  T.  61  N.,  R.  15  W.,  there  is  an  occin-rence 
of  jasper  which  has  been  explored  by  means  of  test  pits  and  diamond  di'ill. 
The  jasper  belt  here  has  a  width  of  from  15  to  30  feet,  strikes  N.  70°  W., 


204  THE  VERMILION  IRON-BEARING  DISTRICT. 

and  dips  90°.  It  is  exposed  for  a  distance  of  about  100  paces  in  an  east- 
west  direction.  South  of  the  jasper  there  is  massive  greenstone.  On  the 
north  side  of  the  jasper  there  is  a  greenstone  which  has  eUipsoidal  parting 
and  i)i  places  appears  tuifaceous.  No  good  sedimentary  banding  is  shown, 
however.  Here,  it  seems,  there  was  an  interbanding  of  the  iron  fomiation 
with  the  greenstones,  it  having  been  covered  up,  perhaps,  by  a  flow  of 
lava  represented  b)^  the  ellipsoidal  and  tuffaceous  rock.  In  the  SE.  ^  of 
the  SE.  J  of  sec.  7,  T.  62  N.,  R.  14  W.,  on  what  is  known  as  the  Eaton 
property,  there  is  a  large  exposure  of  much  plicated  iron  formation  which 
in  one  place  near  the  shaft  has  been  cut  tlu'ough  by  dikes  of  granite- 
porphyry  containing  large  phenocrysts  of  quartz.  The  intrusive  relation- 
ship of  this  porjjhyrv  to  the  jasper  can  be  well  seen  here  The  jasper 
occupies  prominent  hills  in  the  midst  of  an  area  containing  heavy  drift 
deposits  which  conceal  the  greater  portion  of  it,  but  numerous  exposures 
of  greenstone  north  and  northwest  of  the  jasper  areas  indicate  that  the 
greenstone  at  least  partially  surrounded  the  iron  formation,  and,  as  shown 
by  relations  of  these  rocks  elsewhere,  underlies  the  jasper.  Just  west  of 
the  shaft,  about  300  paces  distant,  is  a  bare  knob  of  greenstone  cut  through 
by  a  dike  of  granite-porphyry  which  is  believed  to  be  a  continuation  of 
that  cutting  the  jasper  near  the  shaft.  On  this  bare  knob  there  were 
observed  in  some  places  structures  which  looked  fragmental,  making  the 
rock  appear  as  though  it  were  partially  a  greenstone  tuif  or  a  brecciated 
greenstone.  It  could  not  be  determined  whether  this  fragmental  portion 
of  the  greenstone  was  a  volcanic  tuff'  or  a  basal  conglomerate  lying  upon 
the  greenstone  and  below  the  iron  formation. 

About  one-fourth  of  a  mile  north  of  the  southeast  corner  of  sec.  1, 
T.  62  N.,  R  15  W.,  there  are  a  number  of  fairly  good  exposures  of  the  iron 
formation.  This  is  here  very  intimately  associated  with  greenstone,  with 
which  it  is  clearly  infolded,  both  the  jasper  and  the  greenstone  there  lieing- 
cut  by  acid  dikes.  The  belt  in  which  this  iron-formation  material  occurs 
was  traced  to  the  north  of  east  by  means  of  a  number  of  discontinuous 
exposures  for  about  2J  miles.  There  is  a  large  exposure  just  north  of  the 
little  lake  on  the  section  line,  between  sees.  5  and  6,  T.  62  N.,  R.  14  "W. 
It  is  very  much  contorted,  and  represents  a  southwestward-plunging 
anticline.  Mining  has  been  done  at  this  ])oint  to  a  slight  extent,  some 
ore  having  been  brought  to  the  surface,  although  none  has  been  shipped. 


SOUDAN  FORMATION. 


205 


A  numbei*  of  explorations  farther  east  along  the  same  belt  of  rock  have 
likewise  disclosed  the  presence  of  the  iron  formation,  but  thus  far  only 
very  small  quantities  of  ore  have  been  found. 


Fio.  3. — Sketch  showing  Soudan  formation  infolded  in  Ely  greenstone,  both  cut  by  Keweenawan  dolerite  dike. 

South  of  Moose  Lake  small  stringers  of  jasper  have  been  observed  at  a 
number  of  places,  associated  with  the  Ely  greenstones,  which  are  there  well 
exposed  on  the  bare  hills.  On  the  narrow  strip  of  land  in  the  NW.  5  of 
sec.  4,  T.  63  N.,  R.  9  W.,  sepai-ating  two  small  lakes,  there  are  exposed 
several   narrow    belts    of  iron   formation    intimately   associated    with   the 


206 


THE  VERMILION  IRON-BEARING  DISTRICT. 


greenstone.  Five  distinct  bands  of  iron-formation  material  were  observed, 
each  only  a  few  paces  in  width,  and  in  the  belts  farthest  south  the 
exposures  were  sufficiently  good  to  show  very  well  the  intimate  relations 
of  the   srreenstone   and  the   iron  formation.     The  iron  formation  is  very 


Xf^ 


Ogishke    conglomerate 

I  ron-beanng  formation 
Greenstone 


Scale 
40 


sofeet 


Fig.  4.  — Ilhisiralion  showing  distribution  aiwi  relations  of  Ely  greenstone,  Soudan  I'ormiition.  und  Ogishke  eonylnm- 

erate  at  a  place  south  of  Moose  Lake. 

clearly  infolded  in  the  greenstone,  and  subsequent  truncation  of  these  two 
formations  has  resulted  in  ])roducing  very  intricate  surface  relationships. 
At  one  place  a  broad  dike  of  dolerite  has  cut  across  botli  the  preexisting- 
formations,  and,  indeed,  it  was  traced  tor  idxmt  half  a  mile  across  the 
country.     Fig.  3  illustrates  the  relations   observed  at  this  place.     About 


SOUDAN  FORMATION. 


207 


one-fourth  of  a  mile  due  north  of  this  locality,  in  sec.  33,  T.  64  N., 
R.  9  W.,  there  are  some  exposures  showing  very  clearly  the  relations 
which  exist  between  the  greenstones,  the  iron-bearing  formation,  and  the 
sediments  that  occur  in  such  large  quantities  in  this  area  south  of  Moose 
Lake.  At  this  place  the  greenstones  occupy  the  opposite  sides  of  a 
considerable  depression  in  which  occur  most  commonly  the  exposures  of 
the  fragmentals.  Occasionally  an  irregular  area  of  the  old  ellipsoidally 
parted  greenstone  rises  through  these  fragmental  deposits  (fig.  4).  At 
the  place  mentioned,  a  narrow  belt  of  iron  formation,  about  20  feet  in 
length,  was  observed  lying  on  the  north  side  of  such  a  small  irregular 
area  of  greenstone.  South  of  this 
there  were  small,  irregular  areas  of 
iron  formation  completely  surrounded 
by  the  greenstone.  Immediately  ad- 
jacent to  the  iron  formation  and  the 


OgishUe   conglomerate 


j^i^^Soudan  form  at  I  o  nTircMT^beanrrg) 


N 

A 


greenstone 


occurs   the   Ogishke   con- 


r 


(oreensto  tc 


Sfeet 


Fig.  .5.— Sketch  showing  association  and  relations  of  Ely 
greenstone,  Soudan  formation,  and  Ogishke  conglom- 
erate. 


glomerate,  consisting  of  fragments  of 
greenstone  with,  in  the  immediate 
vicinity  of  the  jasper,  considerable 
numbers  of  fragments  of  jasper,  show- 
ing very  clearly  that  it  is  derived  from 
the  jasper  and  the  greenstone,  which 
are  the  underlvino-  formations.  Fig-. 
5  is  a  sketch  illustrating'  the  associa- 
tion of  the  rocks  at  this  locality.  It 
is  very  evident  that  the  irregular  distribution  of  the  greenstone  is  due  to 
the  intricate  folding  to  which  all  of  the  rocks  have  been  exposed,  and  the 
subsequent  truncation  of  the  folds,  which  has  left  the  outcrops  of  the  forma- 
tions in  their  present  relations. 

On  the  Canadian  shore  of  Otter  Track  Lake,  on  the  west  side  of  the 
strait  leading  to  the  north  and  south  arms,  and  just  beyond  the  main  portion 
of  the  lake,  there  is  a  considerable  exposure  of  iron  formation  in  contact 
with  the  old  greenstones.  The  formations  are  well  exposed  along  the  lake 
shore,  being  present  in  cliffs  which  are  about  75  feet  high  and  very  nearly 
perpendicular.  The  contact  between  the  jasper  and  the  greenstone  is 
very  irregular  on  a  large  scale,  although  when  examined  in  detail  the  line 


208 


THE  VERMILION  IRON-BEARING  DISTRICT. 


is  usually  pretty  sharp.  The  actual  contact  between  the  two  was  closely 
studied  to  find  whether  or  not  a  zone  of  clastic  rocks  intei'vened  between 
them,  but  such  a  zone  was  not  found,  the  contact  being  very  sharp.  The 
greenstone,  it  is  true,  is  somewhat  schistose  along  the  contact,  but  this  may 


Scale 

100        0  100       200      300     AOOfeet 


Fig.  0.— Sketch  sho'wiiig  ri-latiniis  Ijclwi-cn  Sninlnn  fiinniilion  and  Kly  greenstone  vn  otter  Track  Lake. 

be  due  to  the  moA'ement  that  has  taken  place  between  the  tAvo  formations 
as  a  result  of  the  folding.  Fig.  6  is  a  sketch  showing  tlie  occurrence  of 
these  rocks  at  Otter  Track  Lake. 

The  regular  canoe  route  was  followed  nortliward  from  Otter  Track 
Lake  to  Jasper  Lake,  and  on  the  norfli  shore  of  Jasper  Lake,  west  of  the 


SOUDAN  FORMATION. 


209 


bay  from  whicli  the  portage  leads  northward  from  the  lake,  exposures  of 
iron  formation  were  found  and   carefully   stxidied.     Here,   again,    special 


Greenstone  ^.^ ■■ 


Scale 

50  100  feet 


/ 
/ 


Soudan  formation  / 

(iron-bearing)        / 


Fig.  7.— Sketch  showing  relations  of  Soudan  formation  and  Ely  greenstone  on  Jasper  Lake. 
MON  XLV — 03 li 


210  THE  VERMILION  IRON-BEARING  DISTRICT. 

atteutiou  was  paid  to  the  contact  between  the  irou  formation  and  the  green- 
stone, but  very  careful  search  showed  no  indication  of  anj  clastic  material 
between  them.  Nearly  everywhere  this  greenstone  is  perfectly  massive;  in 
a  few  places  it  shows  a  slight  schistosity  parallel  to  the  contact,  probably 
the  result  of  the  shearing  caused  by  the  close  folding.  The  iron  formation 
is  closely  infolded  in  the  syncline  in  the  greenstone.  The  broadest  part  of 
the  syncline — that  is,  the  place  where  the  jasper  has  its  greatest  width — is 
at  the  top  of  the  hill.  From  this  point  down  to  the  lake  shore  the  descent 
is  at  a  rather  sharp  angle,  and  the  fold  has  here  been  beveled  off.  As  a 
result  of  this  beveling  a  subordinate  anticline  is  first  brought  to  the  surface 
as  an  oval  area  near  the  center  of  the  main  syncline,  and  as  we  come  still 
farther  south  the  deeper  truncation  of  the  fold  has  separated  the  main 
syncline  into  two  subordinate  synclines  occurring  in  very  narrow  belts 
separated  by  the  intermediate  greenstone  area,  as  shown  in  fig.  7,  from  a 
sketch  made  in  the  field. 

On  the  portage  between  Big  Rock  Lake  and  Emerald  Lake,  north  of 
the  international  boundary,  a  mass  of  iron  formation  was  seen  which  was 
estimated  to  be  about  100  feet  across,  north  and  south.  Its  contact  with 
the  greenstone  south  of  the  trail  was  found,  and  is  here  knife-like  in  its 
sharpness,  there  being  absolutely  no  clastic  material  between  the  greenstone 
and  the  jasper.  The  jasper  is  much  broken,  has  very  fine  jointing-,  and 
the  brilliant  red  variety  makes  up  the  greater  portion  of  the  formation. 
Some  of  the  bands  of  the  bright-red  jasper  are  6  inches  across.  Bands  of 
white  chert  are  infrequent,  although  present,  and  the  iron  ore  in  narrow 
bands  makes  up  the  remainder  of  the  formation. 

INTRICATE   FOLDIJiTG   OF   SOUDAlSr  FORMATION. 

The  intricate  folding  of  the  iron  formation  can  be  well  seen  at  a 
number  of  places  just  north  of  the  international  boundary;  for  example, 
on  Emerald  Lake,  where  it  is  most  beautifully  and  curiously  folded.  On 
the  west  side  of  the  largest  island,  which  is  about  in  the  center  of  tlie 
lake,  bands  of  the  jasper  are  bent  into  fan-shaped  folds  and  curious 
interlockings,  a  description  of  which  would  be  almost  incredible.  The 
jasper,  although  so  rigid,  has  evidently  obeyed  the  law  of  flowage  in  filling- 
up  every  chink  and  corner  throughout  the  complexly  deformed  mass. 
How  far  these  rocks  were  folded  before  complete  jasperization  took  place 
is  questionable.     At  this  ledge  tlie   l)r()ad  jasper  bands   turn  around   and 


SOUDAN  FORMATION.  211 

form  an  elbow,  having  a  radius  of  from  2  to  4  inches,  giving  a  roundish 
surface  on  top,  looking  like  cross  sections  through  a  set  of  closely  laid 
pipes.  The  minor  folds  at  this  exposure  pitch  to  the  west  at  angles  varying 
greatly,  but  ranging, mostly  between  50*^^  and  30°. 

The  intricate  infolding  of  the  green  schists  and  jas,pers  may  be 
observed  at  numerous  places  on  Lee  and  Tower  hills,  just  north  of  the 
town  of  Tower.  A  study  of  these  will  impress  one  with  the  extraordinary 
complexity  of  this  folding.  Not  only  are  the  dips  substantially  vertical, 
but  the  pitches  of  the  folds  are  vertical,  or  nearly  so.  The  result  of  this  is 
that  where  the  green  schist  and  the  jasper  come  together  the  contact  is 
most  extraordinarily  complex.  It  runs  in  and  out,  and  in  places  the  jasper 
might  be  supposed  to  be  over  the  green  schist;  in  other  places  the  reverse 
seems  to  be  the  case;  and  in  still  other  instances  one  is. strongly  inclined 
to  believe  that  one  of  them  cuts  the  other  as  an  intrusive.  In  some 
places  there  is  brecciation  along  the  contact,  so  that  between  the  green  schist 
and  jasper  there  is  an  intervening  zone  of  pseudo-conglomerates.  One  of 
these  cono-lomerates  has  a  srreen  schist  matrix  in  which  are  bedded  numerous 
fragments  of  banded  jasper.  Some  fragments  are  well  rounded,  others 
subangular,  others  have  curious  points,  and  a  considerable  number  are  in 
roughly  rhomboidal  form  (PL  VI,  C).  The  schistosity  extends  roughly 
east  and  west.  It  cuts  the  schist,  but  usually  stops  abruptly  at  the  jasper 
bands.  This  adds  still  another  feature  to  the  complexity  of  the  structure 
at  the  contacts.  However,  although  one  may  be  confused  by  examining 
the  details  of  some  of  these  exposures,  if  one  follows  the  broad  distribution, 
he  will  find  in  many  places  that  the  schist  and  jasper  occur  in  belts  which 
can  be  separated  as  such,  the  major  folds  being  made  out  in  many  cases. 

One  of  the  largest  exposures  of  jasper  in  the  Vermilion  district  is  that 
which  forms  the  prominent  peak  known  very  commonly  as  Jasper  Peak, 
although  the  name  Chester  Peak  has  prior  claim.  The  jasper  is  exposed 
over  almost  the  entire  area  of  the  hill.  The  outcrops,  while  not  solid,  are 
nevertheless  so  numerous  as  to  enable  one  to  determine  very  easily  the 
structural  features.  The  south  side  of  the  hill  is  nearly  vertical,  and  gives 
very  good  sections  through  the  intricately  folded  iron  formation.  A  study 
of  the  hill  shows  that  the  iron  formation  is  folded  into  a  great  synclinorium 
made  up  of  a  number  of  closely  folded  synclines  and  anticlines.  The  gen- 
eral strike  of  the  axis  of  the  synclinorium  is  N.  60°  E.,  and  the  axis  plunges 
to  the  east,  though  the  exact  angle  is  not  known. 


I 


212  THE  VERMILION  IRON-BEARING  DISTRICT. 

There  are  some  large  exposures  of  jasper  in  the  NE.  \  of  sec.  25, 
T.  63  N.,  R.  12  W.,  and  this  locality  has  been  prospected  thoroughly  by 
means  of  diamond-drill  borings.  The  way  in  which  the  iron-bearing 
formation  is  closel)^  infolded  in  the  Ely  greenstone  is  well  shown  upon 
Sheet  XXVI  of  the  accompanying  atlas,  the  data  for  the  construction  of 
which  have  been  obtained  from  the  Minnesota  Iron  Company. 

CLASTICS  ASSOCIATED  WITH  THE  IROK-BEARIKG  FORMATION. 

There  is  only  one  place  in  the  Vermilion  district  at  which  the  clastic 
■ocks  are  associated  with  the  iron  formation  in  such  a  way  that  no  question 
can  be  raised  that  they  belong  together  and  that  there  is  a  gradation  from  the 
one  into  the  other.  This  place  is  just  south  of  the  north  quarter  post 
of  sec.  7,  T.  62  N.,  R.  13  W.,  on  the  south  slope  of  a  hill.  At  this  place  the 
narrow  bands  of  iron  formation  are  interbanded  with  bauds  of  conglomerate 
and  slate.  There  are  evidently  several  series  of  these  bands,  although  it  is 
possible  that  there  may  be  in  places  a  reduplication  due  to  the  close  folding. 
The  conglomerate  is  made  up  of  fragments  derived  from  the  immediately 
subjacent  greenstones,  and  as  these  fragments  grow  smaller  the  sediments 
grade  upward  through  gray wackes  into  finely  banded  greenish  slates,  next 
to  which  occurs  the  iron-bearing  formation,  consisting  of  chert,  jasper,  iron 
ore,  and  an  occasional  band  of  material  that  may  correspond  to  the  finer- 
grained  slates  adjacent. 

Somewhat  similar  conditions  were  observed  on  the  hill  in  the  NW. 
^  of  the  SE.  -4  of  sec.  10,  T.  62  N.,  R.  14  W.,  Avhich  overlooks  the  large 
swamp  to  the  northwest,  although  the  proof  of  the  relationship  is,  perhaps, 
not  quite  so  clear.  Here  there  is  a  large  exposure  of  jasper  in  which  a  shaft 
has  been  sunk  and  through  which  a  considerable  quantity  of  pyritiferous  ore 
has  been  hoisted.  The  jasper  is  followed  to  the  south  by  a  fragmental  rock 
made  up  of  fragments  of  greenstone.  This  is  about  10  feet  distant  from 
the  jasper.  The  fragmental  rock  is  thoroughl}'  impregnated  with  partial 
pseudomorphs  of  limonite  after  pyrite.  South  of  this  fragmental  occurs  an 
amygdaloidal  lava.  The  sediments  and  the  greenstone  are  both  slightly 
schistose,  the  schistosity  striking  about  east  and  west.  South  and  west  of 
tliis  locality  a  number  of  other  exposures  of  greenstone,  associated  with  a 
fragmental  rock  made  up  of  greenstone  fragments,  was  observed.  In  none 
of  the  sediments  could  well-marked  sedimentary  banding  be  found. 


SOUDAN  FORMATION.  213 

THE  IROlSr-ORE   DEPOSITS. 

HISTORICAL    SKETCH. 

The  first  mention  of  the  occnrrence  of  iron  ore  in  the  Vermilion  dis- 
trict was  made  by  J.  Gr.  Norwood,  who  observed  it  during  his  explorations 
in  1850  and  published  a  statement  concerning  it  in  the  report  accompanying 
that  of  D.  D.  Owen."  The  iron  that  he  observed  is  that  which  occurs  near 
Gunflint  Lake,  at  the  extreme  east  end  of  the  district,  and  which  geologic- 
ally belongs  with  the  ores  of  the  Mesabi  range.  In  this  part  of  the  Ver- 
milion district  the  ores  have  never  been  exploited  to  any  extent  and  are  at 
present  of  no  commercial  importance. 

Interest  in  what  is  now  known  as  the  Vermilion  iron-bearing  district  was 
aroused  in  the  sixties  by  the  rejjorted  occurrence  of  gold  in  the  slates  and 
schists  in  the  region  of  Vermilion  Lake.  There  was  considerable  excite- 
ment for  several  years  and  a  small  rush  to  the  district.  Shafts  were  sunk 
and  stamp  mills  were  erected,  the  machinery  having  been  packed  in  from 
Duluth,  partly  on  the  backs  of  Indian  packers,  over  the  Vermilion  trail.  A 
town  site  was  laid  out  near  Pike  River,  at  the  southwest  extremity  of  Ver- 
milion Lake,  and  some  buildings  were  erected.  In  all  a  good  deal  of  money 
was  fruitlessly  expended,  as  no  gold  deposits  of  any  importance  were  found. 

A  reference  to  the  possible  occurrence  of  hematitic  iron  ore  in  the  Ver- 
milion district,  in  the  strict  sense,  was  made  in  Hanchett  and  Clark's  report 
for  1866.     The  State  geologist  says: 

Specimens  of  hematitic  specular  iron  ore  were  obtained  from  a  heav}^  deposit 
said  to  lay  between  a  lake  forming  the  affluence  of  the  upper  Embarrass  River  and 
Vermilion  Lake.  The  precise  percentage  of  commercially  pure  iron  contained  in 
this  ore  has  not  been  ascei'tained.* 

A  more  detailed  mention  of  the  occurrence  of  the  iron  ore  on  the 
iron  range  at  Vermilion  Lake  was  made  by  H.  H.  Eames,  who  investigated 
this  disti'ict  in  regard  to  the  reported  occurrences  of  gold  and  silver  and 
described  the  iron-ore  deposits  as  follows : " 

«Keport  of  the  Geological  Survej'  of  Wisconsin,  Iowa,  and  Minnesota,  by  D.  D.  Owen,  1852; 
Eeport  of  J.  G.  Norwood,  p.  417. 

f>  Hanchett  and  Clark:  Eeport  of  the  State  geologist,  Aug.  H.  Hanchett,  M.  D. ,  together  with  the 
physical  geography,  metallurgy,  and  botany  of  the  northeastern  district  of  Minnesota,  by  Thomas 
Clark,  assistant  geologist,  St.  Paul,  1865,  p.  6. 

c  H.  H.  Eames,  Report  of  the  State  geologist  on  the  metalliferous  region  bordering  on  Lake 
Superior,  St.  Paul,  1866,  p.  11. 


214  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  iroa  range  of  Lake  Vermilion  is  on  the  east  end  [of  the  lakej.  on  the  stream 
known  as  Two  River,  which  is  about  60  feet  wide.  There  are  two  parallel  ridges 
forming  the  boundary  of  this  stream,  and  at  the  mouth  on  each  side  are  extensive 
tamarack  swamps.  This  range  is  about  one  mile  in  length,  it  then  ceases,  and 
after  passing  through  a  swamp,  another  uplift  is  reached,  from  250  to  300  feet  high. 
The  iron  is  exposed  at  two  or  thi'ee  points  between  50  and  60  feet  in  thickness:  at 
these  points  it  presents  quite  a  mural  face,  but  below  it  is  covered  with  detritus  of 
the  overcapping  rock.  On  this  account  its  exact  thickness  could  not  be  correctlj' 
ascertained.  The  ore  is  of  the  variety  known  as  hematite  and  white  steely'  iron,  and 
is  associated  with  quartzose.  jasperoids  and  serpentine  rocks.  It  genei-ally  has  a  cap 
rock  from  3  to  20  feet  thick.  A  little  to  the  north  of  this  is  an  exposure  of  mag- 
netic iron  of  very  good  quality,  forming  a  hill  parallel  with  the  one  described. 

The  hematitie  iron  has  a  reddish  appearance  from  exposure  to  atmospheric  influ- 
ence; its  fracture  is  massive  and  granular;  color,  a  dark,  steel  gray.  The  magnetic 
iron  ore  is  strongly  attracted  by  the  magnet  and  has  polarity:  is  granularly  massive; 
color,  iron  black. 

The  timber  here  is  very  abundant  and  good,  of  the  same  class  as  prevails  else- 
where in  this  region. 

Some  time  after  this,  in  1875,  the  first  exploratory  work  in  this  district 
was  taken  up  by  Mr.  George  R.  Stuntz,  accompanied  by  Mr.  John  Mall*^ 
mann,  who  began  to  prospect  the  Vermilion  ore  deposits  on  Lee  Hill, 
southwest  of  tlie  bay  of  Vermilion  Lake,  which  is  now  known  as  Stuntz 
Bay,  named  after  Mr.  Stuntz.  In  1880  Prof  A.  H.  Chester  examined 
the  Vermilion  Lake  ore  deposits  for  private  parties,  and  Mr.  Bailey  Willis 
studied  them  for  the  Census  Office.  Systematic  and  extensive  eftbrts  were 
made  in  the  late  seventies  and  the  earl}-  eighties  to  develop  the  iron  resources 
which  were  known  to  be  present  in  this  district.  By  tins  time  the  Minnesota 
Iron  Company  had  been  organized  and  all  of  the  properties  which  at  that 
time  were  known  to  contain  ore  and  great  stretciies  of  country  which  were  in 
the  continuation  of  the  ore  range  had  been  purchased,  the  company  owning 
over  20,000  acres  of  land  on  the  Vermilion  range  proper  and  in  the  vicinity 
of  the  good  harbor  on  Lake  Superior,  known  now  as  Two  Harbors.  On 
August  1,  1884,  the  Duluth  and  Iron  Range  Railroad  was  completed  from 
Two  Harbors  to  Tower.  This  road  was  72  miles  long.  At  a  later  date  it 
was  connected  with  Duluth,  25  miles  away.  Daring  the  first  year  (1884) 
62,122  tons  of  ore  were  shipped,  some  of  this  having  come  from  the  stock 
piles  which  had  been  growing  during  the  years  of  development  jireceding 
the  opening  of  the  railroad. 


SOUDAN  FORMATION.  215 

Prospectors  were  busy  in  the  years  prior  to  the  opening  of  the  raih-oad 
in  prospecting  the  range  to  the  east  of  Tower,  and  in  1883  outcrops  of  ore 
were  found  by  Mr.  H.  R.  Harvey  in  sec.  27,  T.  63  N.,  R.  12  W.  The 
body  of  iron  ore  indicated  by  these  outcrops  was  further  tested  in  1885-86 
and  led  to  the  opening  up  of  the  great  deposits  at  Ely  on  which  are  now 
working  the  Chandler,  Pioneer,  Zenith,  Sibley,  and  Savoy  mines.  During 
1888  there  were  shipped  from  the  Chandler  mine  54,612  tons  of  high- 
grade  ore. 

From  this  time  on  the  development  of  the  range  was  rapid,  as  is  shown 
by  the  annual  increase  in  the  shipments  of  ore  (pp.  242-243). 

ORE  HORIZONS. 

The  iron-ore  deposits  of  the  Vermilion  district  show  a  striking'  analogy 
with  those  of  the  Marquette  district.  Like  them,  they  may  occur  in  two 
positions  with  respect  to  the  iron-bearing  formation.  They  are  found,  first, 
at  the  bottom  of  this  formation  and,  second,  within  it,  the  ores  in  both 
cases  being  the  same  in  character.  The  ores  occurring  at  the  bottom  of 
the  iron  formation  rest  immediately  upon  the  Ely  g-reenstone,  which  thus 
forms  the  foot  wall,  and  are  overlain  by  and  g-rade  up  into  the  jasper  and 
associated  rocks  of  the  iron-bearing  formation,  which  usually  forms  the 
hanging  wall.  Ores  occurring  within  the  formation  either  rest  upon  some 
impervious  part  of  the  formation  above  its  base  or  else  lie  in  the  midst  of 
the  iron-bearing  rocks,  their  position  being  determined  by  certain  factors 
which  will  be  discussed  below.  Workable  ore  deposits  are  known  at  two 
localities — Soudan  and  Ely.  At  Soudan  there  are  a  number  of  deposits 
belonging  together,  so  far  as  mode  of  occurrence  is  concerned,  and  they 
are  worked  through  a  number  of  shafts,  all  belonging'  to  the  Minnesota 
Iron  Company.  With  these  ore  bodies  belongs  that  deposit  of  the  old 
North  Lee  mine  on  Lee  Hill,  near  Tower,  the  ore  of  which  has  long  since 
been  exhausted.  At  Ely  there  are  two  and  possibly  three  ore  bodies  lying 
in  an  approximately  east-west  line,  and  exploited  by  the  Minnesota  Iron 
Company  by  means  of  a  number  of  shafts.  Future  developments  may 
show  that  these  ore  bodies  are  actually  continuous  and  form  one  immense 
body  of  ore. 


216 


THE  VERMILION  IRON-BEARING  DISTRICT. 


THE  ELY  IRON-ORE  DEPOSITS. 
DEPOSITS   OCCURRING    AT    THE    BOTTOM    OF   THE    IRON-BEARING    FORMATION. 

To  this  group  belong  the  deposits  occun-ing  near  Ely,  now  worked  by 
means  of  the  shafts  of  the  Chandler,  Pioneer,  Zenith,  Sibley,  and  Savoy 
mines.     The  jjarticular  deposit  upon  which  the  Chandler  and  Pioneer  mines 


Seal 


oofeet 


No. I  Shaft 


^'g'ifc;«iil^^:::.;\:-;V-':-^v 


•J  -^  •,  •' ',' 


Fig.  8.— Vertical  section  across  Chandler  ore  body  iilongr  line  E-F  of  fig.  9, 

are  Avorking  will  probably  prove  to  be  one  of  the  largest  continuous  bodies 
in  tlie  Lake  Superior  region  (PI.  VIII).  This  particular  ore  body  affords  so 
perfect  a  conhrination  of  the  law  of  the  occui-rence  of  ore  deposits  as  stated 
by  Van  Hise  that  it  is  of  very  great  scientific  as  well  as  economic 
importance,  and  will  therefore  be  described  in  .^ome  detail. 


SOUDAN  FORMATION. 


217 


The  workings  at  the  Zenith,  Sibley,  and  Savoy  have  not  been  extended 
far  enough  to  enable  a  final  statement  to  be  made  as  to  the  shape  of  these 
ore  bodies  or  the  exact  structural  conditions  under  which  they  exist,  or, 
indeed,  to  warrant  a  positive  statement  that  they  may  not  eventually^  be 
found  to  be  connected  with  one  another  and  possibly  also  with  the  Chandler- 


Shaft  NoA^ 


\Shaft  No.2 


55 joofeer 


Fig.  9.— Horizontal  section  through  fourth  and  sixth  levels  of  the  Chandler  mine. 

Pioneer  ore  body.  The  Chandler-Pioneer  ore  body,  as  well  as  the  ore 
bodies  to  the  east,  occurs  at  the  bottom  and  comes  part  way  up  the  sides  of 
a  narrow  canoe-shaped  synclinorium  whose  axis  trends  about  80°  E.  The 
rocks  in  this  vicinity  have  been  very  closely  folded,  and,  indeed,  slightly 
overturned,  so  that  at  the  west  end  of  the  Ely  trough  both  walls  dijD  at  a 
very  high  angle  to  the  north.     As  the  result  of  tinderground  work,  it  is 


218 


THE  VERMILION  IRON-BEARING  DISTRICT. 


found  that  the  south  wall  dips  at  an  angle  of  about  70^  X.  The  north  wall 
ranges  from  a  vertical  position  to  a  dip  of  80°  to  the  north.  However,  this 
northerly  dip  does  not  continue  tliroughout  the  trough,  but,  on  the  contrary, 
it  is  found  to  be  reversed  at  the  east  end  of  the  tr()ugh,  where  the  walls  are 
overturned  in  the  opposite  direction  to  the  walls  at  the  west  end,  and  now 


Scale 

250 


Fip.  10. — Vertical  east-west  section  through  the  Chamller  mine. 

dip  at  a  high  angle — about  80° — to  the  south.  It  is  not  known  exacth' 
at  what  point  in  the  trough  this  change  takes  place.  It  probablv  occurs 
about  the  center  of  the  trough,  near  the  east  side  of  the  Pioneer  jiropertv. 
This  reverse  of  dip  is  possibly  caused  by  the  occurrence  of  a  small  subor- 
dinate cross  anticline,  which  has  produced  a  warping  in  tlie  strata,  with  an 


SOUDAN  FORMATION. 


219 


eastward  pitch.  These  facts  are  shown  by  the  accompanying  iUustrations. 
In  fig.  8,  a  vertical  section  across  the  extreme  western  end  of  the  basin,  there 
is  shown  a  narrow  synchne  with  both  foot  and  hanging  walls  dipping  to  the 
north.  In  fig.  9  there  may  be  seen  horizontal  sections  through  the  fourth 
and  sixth  levels  of  the  Chandler  mine,  with  position  of  section  shown  in 
fig.  1 1 ,  indicated  by  line  A-B.  From  study  of  this  figure  in  comparison 
with  figs.  8,  10,  and  11  we  see  that  farthest  west  and  nearest  the  surface 
the  syncline  is  narrowest,  that  as  we  go  down  we  are  compelled  to  go 
eastward  to  follow  the  base  of  the  ore  body  (which  therefore  pitches 
eastward)  and  that  the  ore  body  widens  very  considerably. 


,_pr/gine/  surface  I  ne 


Open  p 


Fig.  11.— Vertical  section  through  the  Chandler  mine  along  the  line  A-B  of  fig.  9. 

North  of  the  southernmost  narrow  syncline  there  is  an  eastward-pro- 
jecting tongue  of  greenstone  which  indicates  a  subordinate  anticline  with  a 
second  syncline  lying  north  of  it  and  en  dchelon  with  the  southernmost  syn- 
cline. The  workings  of  the  mine  where  they  extend  to  the  bottom  of  the 
ore  deposit  (fig.  11)  show  the  conditions  which  exist  there. 

From  this  section  we  see  that  the  bottom  has  not  a  simple  basin  shape, 
but  in  section  from  north  to  south  shows  several  subordinate  rolls.  These 
are  indicated  also  by  the  irregularities  of  the  western  foot  wall,  which,  instead 


220  THE  VERMILION  IRON-BEARING  DISTRICT. 

of  showing'  a  smooth  surface  of  paint  rock,  projects  to  the  east  in  several 
more  or  less  prominent  tongues.  The  horizontal  plans  of  the  various  levels 
show  that  these  tongues  project  farther  and  farther  eastward  as  the  deeper 
and  deeper  levels  are  reached.  By  the  time  the  eighth  level  is  reached  the 
tongue  there  shown  is  relatively  naiTow.  It  is  possible  that  it  may  die  out 
before  it  goes  much  farther  east,  and  it  is  very  probable  that  another  small 
roll  will  be  found  to  begin  north  of  and  en  echelon  with  it.  This  irregu- 
larity in  the  western  foot  wall  of  the  Chandler  basin  is  well  brought  out  by 
the  mining  operations.  The  di-ifts  which  have  been  put  in  to  open  up  the 
west  end  of  the  ore  body  run  in  the  paint  rock  (altered  greenstone),  and 
have  been  maintained  along  a  course  which  carries  them  approximately 
parallel  with  the  margin  of  the  ore  body.  As  a  result  they  have  a  winding 
course.  One  going  through  these  and  keeping  his  course  may  see  that  he 
follows  the  tongue  to  the  east,  bends  around  this  projection  in  the  paint  rock 
corresponding  to  the  anticline,  and  then  follows  back  west  around  the  suc- 
ceeding syncline  of  ore.  Occasionally  when  the  syncline  of  ore  is  very 
narrow  the  drift  may  cut  directly  across  it,  and  in  such  instances  a  small 
tongue  of  ore  surrounded  by  the  paint  rock  is  shown  in  cross  section. 

The  foot  and  hanging  walls  of  this  overturned  syncline,  as  well  as 
the  western  wall  and  the  bottom  of  the  basin,  so  far  as  is  known  from  the 
mining  work,  are  of  paint  rock  or  soap  rock.  This  soap  rock  is  identical  in 
character  with  the  more  or  less  schistose  amygdaloidal  and  ellipsoidal 
greenstones  which  occur  in  such  abundance  upon  the  sm-face  in  the  vicinity 
of  Ely,  and  are  found  surrounding  the  north,  east,  and  south  sides  of  the 
Chandler  basin  in  the  numerous  exposures.  This  greenstone,  as  has  been 
already  stated,  is  an  altered  basalt  (see  p.  152).  Where  the  greenstone 
lies  in  contact  with  the  jasper  and  ore  it  is  almost  invariably  very  schistose 
as  the  result  of  movements  which  have  taken  place  between  rocks  of  such 
different  physical  characters,  and  which  have  l^een  more  effective  along 
this  plane  than  elsewhere.  Moreover,  since  such  plane  of  contact  rep- 
resents a  directioia  of  easy  flowage  for  the  percolating  watei-s  descending 
from  the  surface,  the  rocks  here  have  been  subjected  to  leaching  and  have 
undergone  very  great  metasomatic  changes.  As  a  result  of  these  changes 
the  ininerals  of  these  rocks  have  in  places  been  altered  to  chlorite,  the 
formation  of  which  has  produced  a  soft  schistose  rock.  The  soft,  soapy 
feel  of  the  rock  causes  it  to  be  spoken  of  by  the  miners  as  soap  rock  or 


SOUDAN  FORMATION.  221 

soapstone.  With  the  other  changes  there  has  ahnost  invariably  been  an 
infiltration  of  iron  oxide  to  a  greater  or  less  extent.  Consequentlv  tlris 
contact  phase  of  the  greenstone  is  usually  impregnated  with  iron  oxide  and 
thereby  colored  red.  To  this  fact  the  rock  owes  its  name  of  paint  rock, 
a  term  which  is  very  generally  used  by  mining  men  for  such  altered 
and  red  rocks.  The  plane  of  contact  between  the  ore  formation  and  the 
greenstone  has  also  been  a  plane  along  which  actual  movement  has  taken 
place,  and  as  a  result  a  zone  of  brecciation  has  been  produced  which 
includes  a  certain  thickness  of  both  the  iron  formation  on  the  one  hand 
and  the  soap  rock  or  greenstone  upon  the  other.  The  thickness  of  this 
zone  varies  greatly.  In  some  places  practically  no  brecciation  has  taken 
place,  but  in  others  there  is  a  considerable  thickness  of  brecciated  rock 
The  production  of  slickensides  along  these  cracks  indicates  movement  even 
in  the  greenstone  at  a  considerable  distance  away  from  the  immediate  line 
of  contact  with  the  iron  formation. 

Immediately  adjacent  to  the  greenstone,  and  showing  with  it  the 
above-described  irregular  contact  surface  due  to  folding-,  lies  the  ore.  Upon 
this  ore  lies  a  capping  of  jasper.  Both  the  ore  and  the  jasper  are  very 
much  cracked,  being-  penetrated  by  innumerable  fractui-es,  as  is  also  the 
greenstone,  though,  as  a  result  of  its  brittle  character,  the  cracks  in  the  iron 
formation  are  far  more  numerous  and  less  continuous  than  those  in  the 
greenstone.  This  fractured  condition  of  the  iron  formation  is  clearly,  due 
to  compression  resulting  from  the  production  of  the  close  synclinal  fold  in 
which  the  formation  lies.  Hence,  since  the  ore  as  well  as  the  jasper  is 
brecciated,  there  is  no  escape  from  the  conclusion  that  the  ore  must  have  been 
formed,  in  great  part  at  least,  prior  to  the  time  of  the  last  folding-  of  the 
district  which  caused  the  fracturing  of  the  rocks.  The  ore  has  been  very 
much  broken  up,  and  this  breaking  has  been  a  g-reat  boon  to  the  mining 
company,  as  it  makes  it  relatively  easy  to  mine.  For  this  reason,  as  has 
already  been  stated,  the  ore  is  frequently  designated  a  soft  ore,  although 
this  is,  in  a  strict  sense,  a  misnomer.  In  reality  the  various  fragments  of  the 
breccia  are  fragments  of  hard  ore.  This  natural  brecciation  has  been  taken 
advantage  of  by  the  efficient  manager,  and  the  system  of  mining  which  has 
been  developed  here  merely  continues  the  natural  process  of  brecciation, 
and  thereby"  the  cost  of  winning  the  ore  is  greatly  reduced.  The  method 
of  mining  employed  in  the  Chandler  is  described  and  illustrated  on  page  240. 


222  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  plane  of  contact  between  the  ore  and  the  jasper  agrees  in  general 
with  the  contour  of  the  basin  (fig.  11).  One  noticeable  feature  is  that  this 
plane  of  contact  in  the  upper  part  of  the  basin  dips  on  the  south  side  to  the 
north,  and  on  the  north  side  of  the  basin  to  the  south.  The  conditions,  in 
other  words,  are  those  of  a  normal  syncline.  As  the  deeper  workings  of 
the  mine  are  reached,  however,  this  dip  is  found  to  be  overturned,  and  upon 
the  north  side  the  dip  of  the  plane  is  to  the  north.  It  will  be  noticed  that 
the  greatest  depth  of  ore  lies,  as  it  normally  should,  in  the  center  of  the 
basin.  The  ore  follows  up  both  the  north  and  the  south  limbs  of  the 
syncline;  it  goes  higher,  however,  and  is  very  much  broader  on  the  south 
side  than  on  the  north  side.  This  condition  of  occurrence  is  in  accord  with 
the  view  held  concerning  the  origin  of  the  ore  deposits,  and  will  be  referred 
to  below.  The  plane  of  contact  between  the  jasper  and  the  ore  is 
irregular  in  the  extreme,  no  sharp  line  of  demarcation  existing  between 
them.  As  has  alreadj^  been  intimated,  the  merchantable  ore  grades  upward 
into  lean  ore,  which  in  its  turn  merges  by  imperceptible  gradations  into 
the  jasper,  with  very  small  quantities  of  interlaminated  iron-ore  bands. 
Bodies  of  jasper  varying  from  minute  pieces  up  to  large  masses  project 
downward  into  the  ore.  Occasionally  a  horse  of  jasper  is  included  in  the 
ore.  Again,  a  body  of  ore  will  project  upward  across  the  line  into  the 
jasper.  Within  this  lean  ore,  or  mixed  ore  and  jasper,  one  can  find  pieces 
of  jasper  showing  pai'tial  change  into  the  ore,  the  banding  of  the  jasper  still 
existing  in  a  more  or  less  perfect  condition  in  the  ore.  There  is  in  reality 
no  sharp  line  between  the  ore  and  the  jasper,  but  there  is  a  gradual 
transition  from  one  to  the  other.  These  facts  will  be  referred  to  again 
under  the  discussion  of  the  orig'in  of  the  ore  deposits,  on  page  230. 

t 

THE  TOWER  AND   SOUDAN   DEPOSITS. 

Let  US  now  consider  the  ore  deposits  at  Tower  and  Soudan.  At  these 
places  the  conditions  of  occiuTence  are  not  nearly  so  simple  as  at  Ely.  In 
the  first  place,  the  iron  formation  in  the  western  part  of  the  district  has  been 
very  much  crumpled,  and  there  appear  to  be  many  alternations  of  jasper  and 
a  schistose  green  rock.  The  relations  of  the  jasper  and  this  green  rock  are 
not  in  all  places  clearly  shown.  At  some  localities  it  appears  that  the  green 
rock  is  a  greenstone  belonging  with  the  Ely  greenstone,  and  hence  the 
basement    on    which  the   jasper  rests.     It  owes  its   alternation  with   the 


SOUDAN  FORMATION.  223 

jasper  to  the  intricate  infolding  and  subsequent  truncation  of  the  two 
formations.  In  other  places  the  evidence  is  fairly  conclusive  that  we  have 
to  deal  with  dikes  which  were  intruded  both  parallel  to  the  banding  of  the 
jasper  and  across  this  banding.  In  both  cases  folding  subsequent  to  their 
intrusion  has  brought  about  structural  relations  similar  to  those  existing 
between  the  jasper  and  the  basement  greenstone.  The  rock  forming  the 
dikes  which  cut  the  iron  formation  is  liow  uniformly  green,  except  where 
discolored  by  the  iron.  Its  chief  component  at  present  is  chlorite,  but 
certain  facts — for  instance,  the  presence  of  quartz  phenocrysts,  which  were 
observed  and  which  will  be  considered  in  detail  at  a  more  fitting  place — 
show  very  conclusively  that  at  least  some  of  these  dikes  were  originally 
acid  intrusives.  The  alterations  that  these  acid  rocks  have  undergone  have 
produced  schistose  rocks,  which  now  are  strikingly  similar  in  macroscopic 
characters  to  some  of  the  greenstones  derived  from  rocks  of  an  originally 
basic  character.  These  dike  rocks  have  been  classed '  as  basic  rocks — 
greenstones.  In  one  case — that  of  the  scliist  a,t  the  Lee  mine — Smyth  and 
Finlay"  state  specifically  that  it  has  been  derived  from  a  quartz-porphyry. 

DEPOSITS    OCCtTREING    AT   BOTTOM    OF   THE    lEON   FORMATION. 

We  have  no  conclusive  evidence  that  any  of  the  deposits  of  iron  ore 
at  Soudan  or  Tower  rest  upon  what  is  the  actual  basement  greenstone. 
Such  an  occurrence,  if  recognizable,  would  be  found  to  be  similar  in  all 
essential  characters  to  the  occurrence  of  the  ore  at  Ely.  We  would  find 
the  ore  at  the  bottom  of  a  synclinal  trough  of  the  iron  formation,  with  the 
greenstone  forming  the  impervious  bottom  and  sides  of  the  trough.  It  is 
impossible  to  recognize  with  certainty  the  true  basement  greenstone  on 
account  of  the  intrusion  of  acid  sills  and  dikes  in  the  iron  formation  in  the 
mines  in  the  vicinity  of  Soudan  and  Tower;  on  account  of  the  intricate 
and  intimate  relationship  existing  between  these  two  kinds  of  rocks,  due  to 
this  intrusion  and  heig'htened  by  the  subsequent  infolding  of  the  eruptive 
and  the  iron-formation  rocks;  and  also  on  account  of  the  resemblance  of 
the  altered  acid  rocks  to  the  greenstone.  The  deposits  at  the  east  end  of 
Soudan  Hill,  which  have  been  mined  out,  are  supposed  to  have  been  laid 

oThe  geological  structure  of  the  western  part  of  the  Vermilion  range,  by  Henry  Lloyd  Smyth 
and  J.  Ralph  Finlay:  Trans.  Am.  Inst.  Min.  Eng.,  Vol.  XXV,  1895,  p.  639. 


224  THE  VERMILION  IRON-BEARING  DISTRICT. 

down  iu  syncliues  upon  the  basement  greenstone,  and  hence  to  belong  to 
the  same  class  of  deposits  as  do  those  occvirring  at  Ely.  The  one  mined 
from  the  old  pit  known  as  the  North  Lee  mine  is  presumed  also  to  have 
been  a  deposit  in  an  analogous  position. 

DEPOSITS   OCCURRING    WITHIN   THE   IRON   FORMATION. 

The  deposits  within  the  iron  formation  may  be  of  two  modes  of 
occuiTence:  (a)  They  may  have  jasper  both  as  a  foot  and  hanging  wall, 
and  hence  may  lie  within  it  and  grade  in  all  directions  into  it  (these 
deposits  are  of  small  size);  or  (&)  the)'  may  have  paint  rock  (soapstone,  or 
soap  rock,  as  it  is  indifferently  called  by  the  miners)  as  foot  wall,  below 
which  is  again  jasper,  with  similar  paint  rock  or  jasper  as  the  hanging  wall. 
Of  this  latter  character,  with  an  occasional  pocket  of  ore  lying  wholly 
within  the  jasper,  are  the  deposits  worked  at  Soudan.  The  ore  deposits 
showing  the  different  modes  of  occuri-ence  just  mentioned  are  of  very 
in-egular  shape  and  varj'  greatly  in  size.  Masses  of  greenstone  project 
from  them  into  the  jasjoer  as  a  result  of  the  irregularities  of  the  foot  wall, 
due  chiefly  to  folding;  or  the  plane  of  separation  between  the  ore  and  the 
jasper  is  very  irregular  and  projections  of  jasper  extend  down  into  the  ore, 
and  the  ore  extends  into  the  jasper,  such  irregularities  rendering  the  mining 
very  uncertain  and  expensive.  Occasionally  great  horses  of  jasper  occur* 
in  the  midst  of  an  ore  deposit. 

The  deposits  occurring  within  the  iron  formation  are  far  less  likely  to 
be  as  large  and  as  continuous  as  those  which  lie  at  the  bottom  of  the  fonna- 
tion  and  rest  upon  an  impervious  basement,  for  the  reason  that  the  deposits 
occurring  within  the  iron  formation- owe  their  existence  to  the  introduction  of 
igneous  rocks  in  the  form  of  sills  or  dikes,  or  to  the  peculiar  conditions  of 
fracture.  Where  the  dikes  are  numerous  there  may  be  a  number  of  rela- 
tively small  deposits  separated  from  one  another  by  intervening  waUs 
(subordinate  dikes)  of  soap  rock  or  paint  rock  of  varying  thickness.  This 
condition  is  well  illustrated  in  the  case  of  the  ore  deposits  on  Soudan  Hill, 
which  are  being  mined  by  the  Minnesota  Iron  Company. 

In  structure  Soudan  Hill  is  a  large  anticline  trending  a  little  north  of 
east  and  pitching  steeply  to  the  west.  The  summit  of  this  anticline  is 
occupied  by  a  syncline  having  the  same  strike  and  pitch  as  the  anticline, 


SOUDAN  FORMATION.  225 

aud  it  is  within  this  syncliue  that  the  deposits  occur.  Before  the  jaspers 
were  as  intricately  folded  as  at  present  they  were  intruded  by  dikes  and 
sheets  of  acid  rocks  similar  in  composition  and  general  character  to  those 
now  outcropping  on  the  islands  and  shores  of  Vermilion  Lake.  These 
sheets  of  igneous  rocks  were  intruded  essentially  parallel  to  the  bedding  of 
the  jasper,  and  were  at  varying  horizons  in  the  jasper,  and  hence  are  sepa- 
rated by  varying  vertical  distances.  The  intrusion  of  these  sheets  has  thus 
divided  the  iron  formation,  as  it  wei'e,  into  a  number  of  bands  of  different 
thickness.  The  dikes  which  cut  through  the  iron  formation  are  of  varying 
trend,  and  these,  as  well  as  the  intercalated  sheets,  were  intruded  in  the 
iron  formation  at  various  angles  with  the  horizon.  When,  aftei-  their 
intrusion,  the  rocks  were  folded,  the  intrusive  sheets  behaved  essentially  as 
intercalated  beds  in  the  iron  formation  and  were  crumpled  with  it  into  close 
synclines  and  anticlines.  When  the  folding  took  place  the  brittle  jaspers 
accommodated  themselves  to  the  movement  by  fracturing,  whereas  the  less 
brittle  eruptive  rock  accommodated  itself  by  shearing. 

Owing  to  the  fractured  character  of  the  associated  iron  formation  the 
downward-percolating  waters  passed  readily  tlu'ough  it,  but  were  stopped 
and  led  along  the  relatively  impervious  acid  igneous  rocks.  These  were 
thus  intensely  affected  by  the  circulation  of  the  water,  which,  bringing  iron 
in  large  quantity  in  solution,  deposited  considerable  quantities  of  it  in  the 
igneous  rocks  during  their  alteration.  As  a  result  of  the  action  of  the 
percolating  waters  these  I'ocks  were  intensely  altered  chemically.  The 
addition  of  iron  rendered  possible  the  formation  of  the  chlorite,  which  is 
not  as  a  rule  characteristic  of  the  alteration  of  acid  rocks.  Now  these 
intrusives  are  essentially  the  same  in  general  appearance  as  the  soapstone 
and  paint  rock  derived  at  various  places  from  the  basic  greenstones.  From 
the  point  of  view  of  their  influence  in  the  formation  of  the  iron-ore  deposits 
they  are  also  absolutely  identical  with  the  above-mentioned  soapstones, 
originally  of  basic  character,  and  they  will  be  designated  as  soap  rock  and 
paint  rock,  in  accordance  with  the  custom  of  the  miners. 

Occurring  in  the  way  described,  in  the  large  central  syncline  of  the 
iron  formation,  these  eruptive  sheets  have  divided  it  into  a  number  of  small 
synclines,  each  with  an  essentially  impervious  basement  of  soap  rock.  The 
sheet  of  rock  forming  the  impervious  bottom  of  one  trough  forms  the 
impervious  top  to  the  next  lower  synclinal  troug-h.  Between  these  lie 
MON  XLV — 03 16 


226  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  intensely  fractured  rocks  of  the  iron  formation.  Through  these  frac- 
tures downward-percolating  water  carried  the  ore  and  deposited  it  upon  the 
impervious  bottom  of  the  troughs  formed  by  the  intercalated  igneous  sheets. 
Hence  it  comes  about  that  the  ore  deposits,  being  derived  from  only  the 
small  quantity  of  iron  formation  which  occurs  between  these  adjacent 
sheets,  are  relatively  small,  and  for  the  most  part  disconnected.  They 
occur,  however,  in  a  syncline,  as  does  the  enormous  Chandler-Pioneer 
deposit  of  Ely,  the  essential  difference  being  that  at  Soudan  the  ore  body 
in  the  syncline  has  Ijeen  separated  into  a  number  of  irregular  bodies,  the 
one  above  the  other,  instead  of  occurring  in  one  continuous  ore  deposit. 

One  might  infer  from  the  above  statement  that  these  sheets  were 
introduced  very  regularly  into  the  iron  formation.  This  would  be  an  incor- 
rect inference.  Anyone  familiar  with  igneous  phenomena  knows  that  the 
dikes  and  sheets  divide  more  or  less  frequently  at  in-egular  iutervahs.  They 
have  done  so  in  Soudan  Hill,  so  that  we  may  find  deposits  joining  other 
deposits  as  a  result  of  the  disappearance  of  the  subdividing  sheet  as  we  go 
away  from  the  point  where  it  leaves  the  main  mass,  or  a  large  deposit  will 
divide  into  two  or  more  small  ones  as  a  result  of  the  introduction  of  siTch  an 
oftshoot  from  a  sill.  Hence  there  may  be  a  very  remarkable  irregularity  in 
the  occurrence  of  the  ore  deposits  formed  in  such  synclinal  basins  where  the 
impervious  bottom  is  due  to  the  presence  of  intrusive  sills.  A  further 
cause  of  irregularity  is  the  introduction  of  the  more  or  less  vertical  dikes 
which  were  contemporaneous  with  the  introduction  of  the  sills.  These, 
cutting  through  both  sills  and  the  associated  iron  formation,  have  still 
further  tended  to  subdivide  the  rocks  into  masses  of  varying  size.  Fur- 
thermore they,  like  the  iron  formation,  were  much  folded,  and  now  occupy 
various  positions  and  are  of  greater  or  less  importance  in  determining  the 
size  of  the  ore  bodies.  Thus,  for  example,  if  a  dike  should  have  cut  across 
a  sheet  at  nearly  right  angles,  and  in  such  a  way  that  when  the  two  were 
folded  a  pocket  with  nearly  imper^'ious  bottom  and  sides  was  formed,  an 
ideal  condition  would  have  been  produced  for  the  deposit  of  ore,  according 
to  methods  described  by  Van  Hise  in  numerous  articles  to  which  reference 
has  already  been  made.  The  larger  the  pocket  the  larger,  other  things 
being  equal,  would  be  the  deposit  of  ore. 

Belonging  with  the  ore  deposits  occurring  within  the  iron  formation 
are  certain  small  deposits  of  relatively  slight  commercial  importance,  but  of 


SOUDAN  FORMATION.  227 

considerable  scientific  interest.  These  deposits  are  those  which  have  the 
fii'St  mode  of  occurrence  mentioned  above.  They  are  surrounded  on  all 
sides  by  jasper.  Hence  their  occurrence  does  not  depend  on  the  formation 
of  a  synclinal  trough  with  impervious  bottom,  as  in  the  previously  con- 
sidered cases.  These  deposits,  as  a  rule,  form  so-called  chimneys  of 
ore,  having  in  general,  as  the  name  indicates,  a  rectangular  outline,  and 
their  origin  is  evidently  due  to  the  presence  of  fracture  planes  or  zones 
in  the  jasper.  Along  these  zones  water  has  percolated  and  has  produced 
the  ore  bodies  from  the  iron  formation  by  the  well-known  process  of 
replacement,  the  ore  diminishing  in  richness  as  the  distance  from  these 
fractures  increases. 

ORIGIN  OF  THE  ORE   DEPOSITS. 

Having  now  described  the  manner  of  occurrence  of  the  ore  deposits 
and  shown  their  relation  to  the  geologic  structure,  we  are  prepared  in  the 
light  of  the  facts  given  to  consider  their  origin.  The  origin  of  ore  bodies 
completely  surrounded  by  jasper  obviously  depends  on  the  occurrence  of 
fractures,  for  the  ore  is  confined  to  the  vicinity  of  the  fractures  and 
diminishes  in  richness  as  the  distance  therefrom  increases.  The  importance 
of  these  fractures  as  channels  for  downward  descending  waters  is  also 
obvious.  Hence  the  connection  between  the  occurrence  of  the  deposits  and 
the  action  of  percolating  water  is  shown.  This  interdependence  is  further 
impressed  upon  one  when  a  study  is  made  of  the  ci'oss  section  of  the  narrow 
ore  deposits  of  Soudan,  and  also  of  the  cross  section  of  the  Ely  trough.  It 
will  be  readily  recognized  that  the  plane  of  contact  between  the  two  forma- 
tions will,  as  a  rule,  perform  the  same  function  as  any  large,  continuous 
fracture  in  the  formation  itself,  in  that  the  plane  of  contact  will  permit  more 
readily  the  passage  of  water,  since  it  has  a  continuous  line  of  weakness, 
than  will  the  small,  discontinuous  fractures  which  may  exist  in  the  forma- 
tion. Hence  the  ore  deposits  in  this  case  will  be  confined  more  or  less 
closely  in  their  occurrence  to  this  plane  of  contact.  Where  the  converging 
streams  of  descending  waters  meet,  at  the  bottom  of  the  trough,  the  action 
has  been  most  intense,  and  consequently  the  largest  bodies  have  been 
accumulated  there.  Theoretical  considerations  show  that  if  such  a  trough  or 
other  favorable  place  is  to  contain  a  large  body  of  high-grade  ore  it  should 


228  THE  VERMILION  IKON-BEARING  DISTRICT. 

be  a  structural  feature  which  has  at  some  point  or  points  exits  for  the 
inflowing  descending  currents  of  water.  Since  the  accumulation  of  the  ore 
depends  on  the  circulating-  waters,  of  which  some  parts  bring  in  and  other 
parts  cause  the  deposition  of  the  iron,  and  since  any  given  amount  of  water 
must  carry  an  exceedingly  small  percentage  of  iron  in  solution,  it  is  evident 
that  the  free  circulation  of  large  quantities  of  water  must  be  an  extremely 
im^jortant  factor  in  the  production  of  an  ore  deposit.  The  second  g-reat 
factor  is  time — a  long  period,  in  which  complete  replacement  may  take 
place,  being  more  favorable  than  a  shorter  one.  Of  course  it  is  here  pre- 
supposed that  there  exist  the  other  conditions  necessary  to  the  accumula- 
tion of  the  ore,  which  are  the  presence  of  ii-on-bearing  material  as  the 
source  of  the  iron  and  the  structural  features  for  its  accumulation. 

A  full  discussion  of  the  chemical  reactions  which  result  in  the  deposi- 
tion of  the  ore  has  already  been  given  by  Irving  and  Van  Hise  in  the  mono- 
graph on  the  Penokee  series,"  and  the  reader  is  referred  to  this  for  the  details. 

The  following-  is  a  summarized  statement  of  Van  Hise's  views  of  the 
general  chemical  process  of  concentration  as  the  result  of  which  this  and 
similar  ore  deposits  have  been  produced: 

The  next  question  to  be  considered  is  the  chemical  process  of  concentration  of 
the  ores.  For  places  where  waters  from  different  sources  are  converged,  this  proc- 
ess has  been  fully  given  in  Monographs  XIX  and  XXVIII  of  the  United  States' 
Geological  Survey.  In  this  paper  the  discussion  will  be  only  summarized.  A  part 
of  the  iron  oxide  of  the  ore  was  deposited  in  its  present  condition  as  an  original 
sediment  containing  silica  and  other  imparities.  However,  the  nature  of  the  sediment 
may  have  been  changed — that  is  to  say,  it  may  have  been  deposited  in  part  as  iron 
carbonate,  or  in  small  part  as  iron  sulphide  or  iron  silicate,  and  later  transformed  to 
iron  oxide  in  situ.  The  lean  material  originalh'  deposited  where  the  ore  bodies  now 
are  has  been  enriched  by  secondary  deposition  of  iron  oxide.  Brietly,  the  process  of 
enrichment  is  believed  to  have  been  as  follows: 

The  source  of  the  iron  for  the  enrichment  of  the  ores  is  believed  to  have  been 
mainly  iron  carbonate.  Meteoric  waters  are  charged  with  oxygen.  As  thej"  enter 
the  soil  the}'  would  be  dispersed  through  innumerable  minute  openings.  The  waters 
which  early  in  their  journey  come  into  contact  with  iron  carbonate  would  have  their 
oxygen  abstracted.  Such  waters  would  be  likeh'  to  be  those  following  circuitous 
routes.  The  deoxidation  of  the  waters  bj-  the  iron  carbonate  would  produce  ferrugi- 
nous slates  and  ferruginous  cherts.     In  this  alteration  the  carbon  dioxide  would  be 

"The  Penokee  iron-bearing  series  of  Michigan  and  Wisconsin,  by  E.  D.  Irving  and  C.  E.  Van 
Hise:  Mon.  V.  S.  (leol.  Survey  Vol.  XIX,  1892,  pp.  283-284. 


SOUDAN  FORMATION.  229 

liberated,  aud  would  join  the  descending  waters.  Thus  carbonated  waters  free  from 
oxygen  would  be  produced.  Such  waters  are  capable  of  taking  a  considerable 
amount  of  iron  carbonate  and  some  iron  silicate  into  solution.  Large  quantities  of 
these  solutions  would  be  converged  upon  the  sides  or  at  the  bottom  of  the  pitching 
troughs,  or  in  other  places  where  there  were  trunk  channels  for  water  circulation. 

After  an  iron-bearing  formation  was  exposed  to  descending  waters  for  a  consid- 
erable time,  a  large  part  of  the  iron  carbonate  adjacent  to  the  surface  would  be 
transformed  to  ferruginous  slates  and  ferruginous  cherts.  This  change  would  take 
place  most  extensively  where  waters  were  abundant  and  a  somewhat  direct  course  led 
to  the  trunk  channels.  After  this  process  was  completed  at  such  places,  the  waters 
now  following  this  direct  route  would  pass  only  through  the  ferruginous  slates 
and  ferruginous  cherts  and  would  reach  the  trunk  channels  charged  with  oxygen. 
There  the  solutions  bearing  iron  carbonate  and  those  bearing  oxygen  would  be  com- 
mingled. Iron  sesquioxide  would  be  precipitated.  Therefore  the  iron  oxide  of  an 
ore  body  consists  in  part  of  iron  compounds  originally  deposited  in  situ  and  in  part 
of  iron  brought  in  by  underground  waters.  The  material  deposited  in  situ  may 
have  been  originally  detrital  iron  oxide  or  it  may  have  been  derived  from  iron  car- 
bonate, iron  sulphide,  or  iron  silicate,  which  was  oxidized  in  place,  or  from  two  or 
all  of  these  sources.  It  has  been  assumed  that  the  part  brought  in  by  underground 
waters  was  mainl}^  transported  as  carbonate,  although  a  portion  may  have  been 
transported  in  some  other  form.  Of  the  two  sources  of  iron  ores,  the  original  material 
and  that  added  by  underground  water,  the  latter  is  upon  the  average  probably  more 
abundant.  But  in  some  exceptional  cases,  where  there  is  a  large  amount  of  detrital 
iron  oxide,  the  material  added  by  underground  waters  may  be  subordinate.  However, 
in  all  cases  it  ma}^  be  said  that  were  it  not  for  the  secondary  enrichment  by  under- 
ground waters,  through  the  addition  of  ii'on  oxide,  the  material  would  not  be  iron 
ore.  The  evidence  of  this  -lies  in  the  fact  that  the  ore  bodies  are  universallj'  confined 
to  the  places  where  underground  waters  have  been  converged  into  trunk  channels. 
The  ore  deposits  contain  upon  the  average  a  less  quantity  of  silica  than  does 
the  average  of  the  iron-bearing  formations.  It  follows  therefore  that  silica  must 
ha/e  been  dissolved.  This  doubtless  was  largely  the  work  of  the  great  volume  of 
water  converged  into  the  trunk  channels.  It  has  been  seen  that  the  waters  which 
carried  iron  carbonate  to  the  ore  deposits  were  carbonated.  The  precipitation  of 
iron  oxide  from  carbonate  liberated  more  carbon  dioxide,  so  that  the  waters  were 
very  heavily  charged  with  carbonic  acid.  In  some  of  tlie  districts  basic  igneous 
rocks  occur  within  the  iron-ore  deposits  or  as  basements  to  them.  In  all  such  cases 
these  basic  rocks  are  found  to  have  lost  a  large  part  or  all  of  their  alkalies.  These 
must  have  passed  into  the  solutions.  Hence  the  waters  moving  along  the  trunk 
channels  would  in  some  cases  contain  alkalies  besides  being  rich  in  carbon  dioxide. 
It  is  well  known  that  such  solutions  are  capable  of  dissolving  silica.  Therefore  the 
conditions  which  result  in  the  precipitation  of  iron  oxide  also  furnish  conditions 
favorable  to  the  solution  of  the  silica.     Silica  is  thus  largelv  dissolved  from  the  ore 


230  THE  VERMILION  IRON-BEARING  DISTRICT. 

bodies  and  transported  elsewhere.  The  removal  of  the  silica  is  ordinarih-  onh'  less 
important  in  the  development  of  the  ores  than  the  addition  of  the  iron.  In  many 
cases  the  abstraction  of  the  silica  proceeded  further  than  the  deposition  of  the  iron 
oxide,  thus  making'  the  rocks  verA^  porous  and  further  rendering  the  conditions 
favorable  for  abundant  circulation." 

As  the  result  of  the  detailed  study  of  the  ores  in  the  various  Lake 
Superior  iron-bearing  districts  the  conclusion  has  been  reached  that  they 
are  essentially  replacement  deposits,  and  this  conclusion,  pronounced  early 
in  the  study  of  the  district,  has  been  strengthened  by  the  observation  of 
numerous  facts  in  all  the  other  districts  which  have  been  studied  since  then. 
The  following  facts  from  the  Vermilion  district  alone  seem  to  offer  incon- 
testable proof  that  this  is  the  character  of  the  deposits  in  this  district,  and 
also  gives  clear  proof  of  the  time  of  the  formation  of  these  deposits.  Near 
the  west  end  of  the  ore  body  worked  from  shafts  No.  7  and  No.  8  on  Soudan 
Hill  there  is  a  large  mass  of  jasper,  already  described  by  Smyth  and  Finlay,'' 
lying  directly  across  the  <5re  with  banding  con-esponding-  to  and  continuous 
with  the  banding  of  the  adjacent  ore.  Again,  on  Lee  Hill,  along  the  south 
and  west  sides  of  the  old  North  Lee  mine,  a  breccia  between  the  iron 
formation  and  the  underlying  schist  has  been  produced.  Some  of  the 
fragments  of  jasper  in  this  breccia  have  been  replaced  by  hematite  with, 
however,  a  j^fii'tial  retention  of  the  banded  structure  of  the  jasper.  That 
the  iron  ore  at  this  particular  place  was  deposited  later  than  the  movement 
which  formed  this  breccia  is  shown  by  the  fact  that  the  fragments  of  the 
breccia  are  cemented  in  many  places  by  hematite  ore,  and  numerous  similar 
bodies  may  be  seen  which  have  been  formed  in  cavities  within  the  breccia. 
A  study  of  the  ore  remaining  in  place  at  the  North  Lee  mine  and  the 
adjacent  banded  iron  formation  also  shows  intimate  connection  between  the 
two.  The  iron  formation  has  an  approxiiuately  east-west  trend  and  seems 
to  rest  in  a  westward-pitching  trough  of  chloritic  schist,  the  schist  occurring 
both  on  the  north  and  south  as  well  as  at  the  east  end  of  the  iron  formation. 
The  ore  body  corresponds  in  trend  with  the  strike  of  tlie  formation  itself, 
and  on  its  south  and  west  sides  lies  next  to  the  iron  formation,  the  banded 
jaspers.  As  the  ore  body  is  followed  westward,  the  ore  is  gradually  more 
and  more  mixed  with  jasper,  becoming  lean  ore,  and  then  the  stringers  of 


«The  iron-ore  deposits  of  the  Lake  Superior  region,  by  C.  R.  Van  Hise:  Twenty-tirst  .'V.nn. 
Kept.  U.  S.  Geol.  Survey,  Pt.  Ill,  1901,  pp.  326-328. 
iOp.  cit.,  fig.  8,  p.  42. 


SOUDAN  FORMATION. 


231 


ore  continue  into  the  jasper,  gro"wer  fewer  and  thinner,  until  finally  the  iron 
formation  consists  almost  exclusively  of  jasper  and  chert,  with  but  isolated 
narrow  layers  of  ore  in  it.  The  banding  of  the  jasper  is  seen  to  be  con- 
tinuous with  that  of  the  ore,  which  still  possesses  a  banding,  though  an 
imperfect  one. 

At  one  place  on  Soudan  Hill,  north  of  open  pit  No.  6,  a  contorted 
banded  iron  formation  is  cut  by  a  dike  which  runs  nearly  north  and  south, 
cutting  across  the  bands  of  the  formation  (fig.   12).     On  the  east  side  of 


Fig.  12. — Reproduction  of  sketch  showing  replacement  oi  jasper  by  iron  ore.    After  Smyth  and  Finlay." 

the  dike  and  between  the  dike  and  the  jaspers  and  cherts  there  has  been 
formed  a  small  ore  deposit.  Here  the  banding  in  the  adjacent  jaspers  and 
cherts  appears  to  run  right  on  through  the  ore,  and  although  interrupted  by 
the  dike  is  found  to  be  continuous  beyond  this. 

At  Ely  the  rock  has  been  very  much  brecciated,  but  even  there  the 
banding  in  the  ores  and  their  intimate  mixture  with  the  bands  of  jasper 
seem  to  show  very  conclusively  that  their  relation  is  essentially  the  same 
as  that  of  the  ores  and  jaspers   at  Tower  and   Soudan  which  have  been 


a  Reproduced  from  The  geological  structure  of  the  western  part,  of  the  yermilion  range,  Minne- 
sota, by  Smyth  and  Finlay:  Trans.  Am.  Inst.  Min.  Eng.,  Vol.  XXV,  October,  1895,  p.  643. 


232  THE  VERMILION  IRON-BEARING  DISTRICT. 

described  above.  The  only  explanation  as  to  the  origin  of  the  ore  which 
ajjpears  to  conform  at  all  to  the  facts  is  that  the  ore  is  the  result  of  a 
process  of  replacement,  and  that  the  original  rock  was  a  banded  rock, 
either  essentially  the  same  as  the  present  banded  jasper  or,  as  seems 
more  likely,  the  same  kind  of  rock  as  that  from  which  the  jasper  itself 
has  been  derived  by  replacement.  The  presumed  original  nature  of  this 
rock  has  already  been  discussed  (p.  191)  and  the  conclusion  reached  that 
it  was  a  cherty  iron  carbonate  essentially  similar  to  that  described  from 
the  various  iron-bearing  districts  of  Michigan,  and  especially  from  the 
Penokee-Gogebic  district  of  Wisconsin  and  Michigan. 

An  explanation  of  the  ore  as  a  chemical  deposit"  contemporaneous  with 
the  deposition  of  the  remaining  portions  of  the  iron  formation  is,  as  Smj^th 
and  Finlay  have  already  stated,  altogether  incompatible  with  the  .occurrence 
described  above. 

In  the  case  of  every  known  bod}-  jasper  forms  at  least  one  boundary  in  some 
part  of  it,  under  such  circumstances  that  the  bands,  if  continued,  would  run  into  the 
ore.  This  fact,  taken  in  connection  with  the  tortuous  form  of  many  of  the  bodies, 
seems  to  us  quite  inexplicable  on  any  theory  of  contemporaneous  deposition  of  jasper 
and  rich  ore.  For  such  a  theory  would  involve  the  extraordinary  assumption  that 
the  conditions  of  sedimentation  or  chemical  precipitation  were  so  radically  different 
on  opposite  sides  of  an  imaginary  vertical  plane  in  ocean  water  as  to  permit  the  con- 
temporaneous deposition  or  precipitation  of  nearly  pure  silica  on  one  side  and  nearly 
pure  ferric  oxide  on  the  other,  and  that  such  differences  in  conditions  persisted  long 
enough  to  permit  the  accumulation,  in  some  cases,  of  100  feet  or  more  of  material.* 

The  theory  that  the  ore  is  primarily  the  result  of  replacement  b}^  iron 
oxide  of  various  substances,"  notably  iron,  calcium  and  magnesium  carbon- 
ates, and  siHca,  and  of  accumulation  of  the  replacement  products  in  places 
especially  suitable,  the  location  of  these  places  being  due  to  geologic 
structure,  is  in  direct  accord  with  the  facts  whicli  have  been  observed  in  the 
district,  and  which  have  already  been  discussed  in  considerable  detail. 

The  time  of  the  accumulation  of  the  ore  can  be  fixed  approximately. 
It  was  subsequent  to  the  folding  which  produced  the  synchnal  troughs 
in  which    the    ores    are  now  formed.      This   folding   was  of  course  also 


«The  iron  ores  of  Minnesota,  by  N.  H.  and  H.  V.  Winchell:  Geol.  and  Nat.  Hist.  Survey  of  Min- 
nesota, Bull.  No.  6,  1891,  pp.  103-112;  N.  H.  Winchell,  Geol.  and  Nat.  Hist.  Survey  of  Minnesota, 
Final  Rept.,  Vol.  IV,  1899,  p.  547. 

''Smyth  and  Finlay,  op.  cit.,  pp.  643-644. 

cMon.  U.  S.  Geol.  Survey  Vol.  XXVIII,  1897,  pp.  400-40.5. 


SOUDAN  FORMATION.  233 

partly  instrumental  in  fracturing  the  brittle  iron  formation  and  rendering 
it  thereby  more  permeable  to  percolating  water,  which  was  the  agent 
which  effected  the  accumulation.  A  consideration  of  the  above  fact 
further  strengthens  the  theory  that  the  folding  and  fracturing  preceded 
the  accumulation  of  the  ore.  That  this  accumulation  clearly  took  place 
subsequent  to  this  fracturing  is  furthermore  proved  by  the  fact  that  the 
fractures  which  traverse  the  iron  formation  have  been  frequently  cemented 
by  infiltrated  iron  ore. 

In  the  case  of  the  Tower  and  Soudan  deposits  there  appears  no  evi- 
dence of  folding  subsequent  to  the  accumulation  of  the  ore  deposits,  for  the 
ore  is  uniformly  fairly  massive;  although  such  folding  occurred.  Moreover, 
we  do  not  find  in  these  deposits  the  micaceous  hematites  or  schist  ores  which 
ai-e  found  occasionally  in  the  Marquette  district  of  Michigan,  and  which  owe 
their  origin  to  the  shearing  to  which  they  were  subjected  while  they  were 
so  deeply  bui'ied  that  they  were  essentially  in  the  zone  of  flowage  and  did 
not  undergo  fracturing,  svich  as  is  produced  under  ordinary  conditions. 

The  case  is  somewhat  different,  however,  at  Ely.  There,  it  is  certain, 
more  or  less  extensive  earth  movements  took  place  after  the  ore  was  depos- 
ited, for  the  ore  and  the  overlying  jasper  are  fractured  through  and 
through,  so  that  they  resemble  in  places  a  breccia ;  and  since  these  ores  are 
very  thoroughly  fractured,  we  conclude  that  the  movement  to  which  they 
owe  this  fracturing  took  place  while  the  ores  were  relatively  near  the 
surface,  or,  in  other  words,  were  in  the  zone  of  fracture  for  the  ore  and  the 
associated  jaspers  Had  they  been  more  deeply  buried,  micaceous  hematites 
would  have  been  produced,  and  the  cost  of  exploitation  would  have  been 
very  much  greater  than  it  is  at  present.  As  it  is  now  the  ore  is  almost  a 
rubble,  and  can  be  mined  much  more  economically  than  can  the  massive 
ore  at  Tower  and  Soudan.  It  is  interesting  to  note  that  subsequent  to  the 
formation  of  this  rubble  the  ore,  at  least  at  the  extreme  east  end  of  the  Ely 
trough,  has  been  cemented  together  by  infiltrated  material — iron  ore  to  a 
certain  extent,  but  also  calcite  and  siderite  to  a  still  greater  extent.  Where 
this  cementation  of  the  brecciated  ore  has  taken  place,  as,  for  instance,  in 
the  Savoy  mine,  the  ore  is  almost  as  hard  as  that  obtained  from  the  Soudan 
mines. 

From  the  above  statements  the  impossibility  of  fixing  the  time  of  the 
formation  of   the   ore   deposits  very  definitely  will  be  recognized.     The 


234  THE  VERMILION  IRON-BEARING  DISTRICT. 

process  of  folding  was  inaugurated  between  Archean  and  Lower  Huronian 
time;  but  since  the  present  attitude  of  the  troughs  in  which  the  main  ore 
deposits  are  located  was  mainly  produced  by  the  folding  of  the  Lower 
Huronian,  the  replacement  certainly  occurred,  for  the  most  part,  after 
Lower  Huronian  time.  Since  there  are  no  pre-Cambrian  deposits  later 
than  the  Lower  Huronian  in  this  part  of  the  district,  the  determination  of 
the  time  of  the  replacement  process  can  not  be  more  accurately  made.  The 
process  begun  shortly  after  Lower  Huronian  time  doubtless  has  continued, 
perhaps  with  interruptions,  to  the  present  time. 

METHODS   OF  MINING  IN   THE  VERMILION  DISTRICT. 

All  of  the  ores  of  the  Vermilion  district  are  at  present  obtained  by 
means  of  underground  workings.  The  underground  work  follows  one  of 
two  systems — either  that  known  as  the  "overhand  stoping"  system,  or 
that  known  as  the  "caving"  system.  Both  systems  have  been  modified  in 
certain  particulars,  according  to  the  peculiarities  of  the  deposit  or  in 
ao-reement  with  the  ideas  of  the  management  as  to  the  most  economical 
methods  of  exploitation.  It  would  therefore  be  impossible  to  give  in  this 
place  detailed  descriptions  of  all  the  methods  in  use,  as  this  would  involve 
practically  a  description  of  the  system  followed  in  every  mine  in  the 
district;  but  a  brief  space  will  be  devoted  to  a  description  of  the  methods 
used  at  two  of  the  typical  deposits — the  mines  at  Soudan,  which  use  a 
system  of  overhand  stoping,  and  the  Chandler  mine,  of  Ely,  which  is 
exploited  by  means  of  the  caving  system. 

In  the  mines  at  Soudan  the  system  of  overhand  stoping  is  best 
developed,  and  therefore  these  mines,  and  especially  the  workings  of  No. 
8  shaft,  may  be  regarded  as  a  type  of  this  system.  The  description  which 
follows  is  in  part  an  abstract  of  papers  b}'  Bacon"  and  by  Denton''  and  of 
the  statements  of  Mr.  F.  Ahbe,  sometime  mining  engineer  in  charge  of  the 
mine,  and  in  part  is  the  result  of  personal  observation  hj  the  author. 

The  ore  at  Soudan,  which  is  a  hard,  blue  hematite,  occurs  in  great 
irregular  bodies  of  more  or  less  lenticular  shape,  which  dip  steeply  to  the 


"Development  of  Lake  Superior  iron  ores,  by  D.  H.  Bacon:  Trans.  Am.  Inst.  Min.  Eng.,  Vol. 
XXI,  1892,  pp.  299-304. 

(<  Elements  of  methods  of  metal  mining  based  upon  Lake  Superior  practice,  by  F.  W.  Denton: 
Engineers'  Year  Hook,  University  of  Minnesota,  Vol.  IV,  1896,  pp.  49-67. 


SOUDAN  FORMATION. 


235 


north  at  angles  ranging  from  65°  to  80°,  and  pitch  to  the  west  at  an  angle 
of  22°  to  45°.  As  the  result  of  this  pitch,  the  deepest  levels  are  farthest 
west.  The  deepest  shaft, 
No.  8,  was  down  926  feet 
at  the  thirteenth  level  in 
1 902.  Fig.  1 3,  taken  from 
Bacon's  account  referred 
to  above,  is  a  cross  sec- 
tion through  shaft  No.  8, 
showing"  the  condition  of 
the  Minnesota  Company's 
mine,  presumably  about 
1893,  the  time  of  the  pub- 
lication of  the  article.  It 
shows  the  general  arrange- 
ment of  the  workings.    The 


ore  bodies  lie  one  above 
the  other,  and  are  nsuallv 
separated  from  one  another 
by  impervious  basements 
of  "paint  rock"  or  "soap 
rock."  Sometimes  they 
are  partially  surrounded 
by  material  of  the  iron 
formation  proper,  that  is, 
the  jaspers,  cherts,  and 
interbedded  bands  of  hem- 
atite. AVhen  first  opened 
up,  the  ore'  deposits  were 
exploited  by  means  of 
open  pits.  These  were 
carried  down  to  a  maxi- 
mum depth  of  150  feet, 
when  it  was  found  advisable  to  begin  underground  mining.  From  a  shaft 
in  the  soap  rock  which  forms  the  foot  and  hanging  walls,  crosscuts  are  rui 


Fig.  13.— Cross  section  at  No.  8  shaft,  Soudan,  Minn. 


236 


THE  VERMILION  IRON-BEAKING  DISTRICT. 


off  at  levels  about  75  feet  apart.  From  such  a  crosscut  a  slice  of  ore 
extending  from  the  foot  to  the  hanging  wall,  and  from  15  to  20  feet  thick, 
is  removed  or  stoped  out.  When  this  has  been  cleared  out,  drift  sets 
consisting  of  legs  9  feet  long,  and  averaging  15  inches  in  minimum 
diameter,  with  caps  11  feet  long  and  averaging  16  inches  as  the  minimum 
diameter,  with  heavy  lagging,  are  set  up,  running  from  the  crosscut 
through  the  stope,  usually  near  its  center.  PI.  IX,  A,  shows  these  main- 
level  timbers  being  put  in  on  the  floor  of  the  stope.  After  the  timbering 
is  completed,  filling  is  begun.  These  drift-set  timbers  are  apparently  fully 
strong  enough,  as  80  feet  of  rock  which  is  present  over  some  of  the  drifts 
has  not  broken  them.  Fig.  14,  a  horizontal  section  through  the  fifth 
level  of  the  Minnesota  mine  near  its  connection  with  the  shaft,  shows  the 


Fig.  14.— Horizontal  section  through  the  fifth  level  of  No.  8  shaft,  Soudan. 

arrangement  of  the  drifts  and  their  connection  with  the  crosscut  in  the 
foot  wall  connecting  them  with  main  shaft.  On  the  main  level  the  ladder 
ways  and  chutes  or  mills,  6  feet  square,  requiring  timbers  7  feet  long  and 
averaging  12  inches  in  diameter,  with  a  minimum  of  9  inches  in  diameter, 
are  timbered  up  a  few  feet  above  the  drift  sets.  The  space  from  which  the 
ore  has  been  removed  is  then  filled  and  the  drift  sets  covered  with  several 
feet  of  rock.  Fig.  15  illustrates  the  way  in  which  these  fills  are  made,  and 
shows  how  connection  is  maintained,  by  means  of  chutes  and  ladder  ways, 
with  the  inain  drifts  at  the  bottom  of  the  level. 

PI.    IX,    B   shows    the    top    of    one    of    the    fills    in    the    Minnesota 
Company's  mine.     In  the  background  is  seen  the  cribbed  jiortion  of  the 


U.    S.   GEOLOGrCAL   SURVEY 


MONOGRAPH    XLV      PL.    IX 


.1.      MAIN-LEVEL  TIMBERING,    MINNESOTA    MINE. 


'^:^^M 


B.     FILLING   SYSTEM,    MINNESOTA    MINES,   SOUDAN,    MINN.,    WITH    CHUTE   FOR    DISCHARGING    REFUSE    FROM    UPPER 

LEVELS. 

From  photographs  belonging  to  the  Minnesota  School  of  Mines. 


SOUDAN  FORMATION. 


237 


raise,  which  passes  tlu'ough  the  level  and  from  which  rock  for  the  filling 
is  obtained,  with  a  loaded  tram  car  below  its  mouth.  This  raise  extends 
upward  through  the  foot  wall,  with  ore  forming  the  front  side  of  the  raise, 
to  the  level  above,  through  which  it  passes  in  a  cribbed  way,  and  so  on  to 
the  surface.     Below  the  point  shown  in  the  figure  the  raise  is  cribbed,  and  is 


Oi.i'.'o.'^ 


?  -^ 


BOTTOM  or  PIT    „■■  !       * 


-^^mmzm^^^^^^^' 


\\4-TH  L^VEL 


I  5TH  LEVEL 


eTH  LEVEL 


80  feet- 


Fig.  15.— Longitudinal  section  through  Soudan  mine. 

carried  by  this  cribbing  tlii-ough  the  fills  on  the  various  levels  below.  To 
the  left  is  shown  a  diamond  drill,  used  in  drilling  holes  in  the  ore  fonning 
the  roof  of  the  stope,  into  which  dynamite  is  then  introduced  and 
discharged,  bringing  down  enormous  masses  of  the  ore. 

The  rock  -for  this  filhng  is  obtained  from  the  raises  which  are  cut  in 
the  soap  rock    at    the    foot     or   hanging    wall,   and  which    communicate 


238 


THE  VERMILION  IRON-BEARING  DISTRICT. 


from  level  to  level  with  the  bottom  of  the  open  pit.  Such  a  raise  passes 
through  a  level  with  its  fi-ont  face  cribbed  and  an  opening  in  the  raise  at 
the  height  at  which  it  is  desired  to  discharge  the  filling  into  the  trams 
waiting  for  it.     When  filling  is  desired  at  any  place  the  entire  raise  below 

that  point  is  filled,  and  the  filling 
then  run  through  the  opening  in  the 
cribbing  at  the  proper  place;  or  tim- 
bers and  rails  may  be  laid  across 
the  raise,  making  a  floor,  which  pre- 
vents the  filling  descending-  below 
that  point,  and  from  this  place  it  is 
then  run  out  into  the  trams. 

Rock  to  fill  the  topmost  level, 
or  any  other  level  being  worked  at 
the  same  time  with  it,  is  taken  from 
the  accumulations  of  loose  material 
at  the  bottom  of  the  open  pit,  de- 
rived from  the  caving  in  of  its  sides 
and  from  the  drift  at  its  surface,  or 
else  from  material  which  has  been 
blasted  down  from  the  walls  of  the 
pit.  After  the  topmost  level  is 
worked  out  filling  may  be  run  down 
into  the  level  below  that,  and  so  on 
down  as  the  development  requires  it. 
Fig.  16  is  a  cross  section  through  the 
Minnesota  mine  showing  how  the 
raise  in  the  foot  wall  connects  with 
the  surface,  and  how  connection  is 
maintained  with  the  levels  at  various 
heights  as  the  filling  proceeds. 

Upon  this  filling  the  workmen 
stand,  and  here  the  (frills  are  placed,  mounted  on  braced  cribbing,  or  in 
any  other  way  that  will  give  them  ;i  sufiiciently  firm  foundation.  This 
keeps  the  workmen  near  the  roof,  which  is  one  of  the  advantages  of  oA-er- 
hand  stoping,  in  that  the  roof  can  be  easi'y  examined  and  accidents  from 


Ifi. — Cross  section  of  Soudan  mine  show-ins  raise. 


U.   S.   GEOLOGICAL   SURVEY 


MONOGRAPH    XLV      PL. 


A.     VIEW   SHOWING    METHOD    OF    LOADING   CARS. 


B.     VIEW   OF    MAIN    DRIFT   WHICH    HAS    BEGUN   TO   CAVE.   CHANDLER    MINE 
From  photographs  belonging  to  the  Minnesota  School  of  Mines. 


SOUDAN  FORMATION.  239 

falls  prevented  by  breaking-  down  tbe  rock  as  soon  as  it  is  observed  to 
be  loose.  From  the  top  of  the  fill  the  roof  can  readily  be  reached  and  a 
slice  of  ore  about  10  feet  in  thickness  is  blasted  down  from  it,  broken 
up,  and  tlu'own  into  the  chutes  (mills).  These  chutes  are  25  feet  apart. 
Formerly  they  were  much  more  widely  separated,  but  experience  has 
shown  that  the  distance  now  maintained  is  the  best.  The  men  han- 
dling the  ore  which  falls  between  can  work  both  ways  from  the  chutes 
and  can  throw  the  ore  into  the  chutes  without  tramming  or  second 
handling.  These  chutes  lead  down  to  the  drift  below,  and  from  them 
tlie  ore  is  let  out  into  the  tram  cars.  PI.  X,  A  shows  the  method  of 
loading  these  cars  in  the  main  drift  at  the  bottom  of  one  of  the  chutes. 
When  filled  the  cars  are  hauled  by  mules  to  the  shaft  and  thence  hoisted 
to  the  surface.  As  the  stope  is  extended  filling  is  let  in  from  the  raises, 
and  the  chutes  and  ladder  ways  are  extended  upward  to  keep  pace  with 
the  filling. 

The  ore  is  very  hard  and  the  cost  of  breaking  it  is  high.  Percussion 
or  power  drills  are  used  to  some  extent,  but  diamond  drills  are  more  com- 
monly used  for  boring  the  holes  which  are  used  in  blasting  the  ore  down. 
This  is  probably  the  only  mine  in  the  district  in  which  the  diamond  drills  are 
used  for  boring  preparatory  to  blasting.  It  has  been  found  that  diamond 
drills  are  in  the  long  run  cheaper  for  this  work  than  percussion  drills. 

After  having  been  hoisted  to  the  svirface  the  ore  is  run  tlii'ough  Blake 
crushers,  which  reduces  it  to  sizes  suited  for  furnace  use.  The  ore  is  then 
run  directly  into  cars  d;iring  the  shipping  season,  or  during  the  winter 
season  is  piled  in  stock  piles  and  loaded  from  these  by  steam  shovels  into 
cars  when  the  shipping  season  begins. 

The  ore  deposits  at  Ely  are  the  most  important  and  most  interesting 
in  the  district.  The  oldest  and  most  productive  mine  at  Ely  is  the 
Chandler,  in  whicli  the  ore  is  mined  on  the  caving'  system.  The  great 
body  of  ore  that  is  exposed  by  the  Chandler  lies  in  a  trough  of  greenstone 
which  plunges  to  the  east  at  an  angle  of  about  45°.  The  continuation  of 
this  same  ore  body  is  worked  on  the  caving  system  by  the  Pioneer  mine  to 
the  east  of  the  Chandler,  which,  since  it  is  on  the  pitch  of  the  ore  body, 
gets  all  of  the  water  from  the  Chandler.  A  layer  of  sheared  greenstone 
discolored  by  iron  (paint  rock),  20  to  22  feet  thick,  lies  between  the  ore  and 
the  comparatively  massive  greenstone.     The  foot  and  hanging  walls  of  this 


240  THE  VERMILION  IRON-BEARING  DISTRICT. 

paint  rock  dip  to  the  north  at  an  angle  of  about  70^.  Figs.  10  and  11  are 
respectively  vertical  E.-W.  longitudinal  and  vertical  N.-S.  cross  sections  of 
the  Chandler  mine  showing  the  features  here  described.  The  ore  is  capped 
by  fractured  iron-formation  material,  jasper,  chert,  and  ore  bands,  which  is 
overlain  by  glacial  drift.  The  ore  body  has  been  reached  through  five 
shafts  sunk  in  the  greenstone.  The  greatest  depth  is  attained  by  No.  5  shaft, 
which  was  down  in  1902  to  the  eighteenth  level,  at  a  depth  of  740  feet 
vertically.  All  of  the  shafts  were  originally  vertical,  but  shafts  Nos.  2  and 
4  were  sunk  so  close  to  the  ore  body  that  the  upper  portion  slid  into  the  pit 
as  the  result  of  the  caving.  Inclined  shafts  with  openings  farther  back 
from  the  pit  were  sunk  to  intercept  the  shafts  at  a  point  where  the  caving 
has  not  injured  them.  Most  of  the  work  is  now  done  from  shafts  Nos.  3 
and  5.  The  method  of  mining  is  the  caving  system,  slightly  different  in 
the  newer  woi'kings — that  is,  in  the  lower  levels — from  what  it  is  higher 
up.  The  following  concise  description  by  Denton  will  give  an  idea  of 
the  system:" 

Down  to  the  eighth  level  the  method  of  mining  is  as  follows:  Main  levels  are 
driven  75  feet  apart  and  generally'  there  are  two  main  drifts  at  the  bottom  of  each 
block,  running  approximately  parallel  on  opposite  sides  of  the  block  of  ore.  From 
these  main  drifts  raises  are  put  up  at  intervals  of  about  50  feet,  and  from  these  raises 
four  series  of  subdrifts  are  run.  The  sets  in  the  main  drifts  are  made  of  9-foot  caps 
and  7-foot  legs,  and  those  in  the  "subs"  of  6-foot  caps  and  6-foot  legs.  This  leaves 
about  8  feet  of  ore  between  the  sublevels.  The  sets  are  placed  3  to  i  feet  apart. 
As  the  raises  are  put  up,  sets  are  placed  to  start  the  first  subdrifts:  but  these  drifts 
are  not  run  at  once,  but  are  omitted  to  strengthen  the  main  drifts  until  the  fourth, 
third,  and  second  "subs"  have  been  worked  out.  When  the  subdrifts  are  completed, 
the  block  of  ore  between  any  two  levels  is  honej'combed  with  drifts  with  vertical 
intervals  of  8  feet  of  ore.  When  mining  above  has  been  completed,  the  removal  of  the 
ore  pillars  on  the  top  "subs"  begins.  The  pillars  are  sliced  away,  the  back  is  caved, 
and  the  caved  ore  is  removed  in  wheelbarrows  to  the  chutes  leading  to  the  main  level 
below.  The  chutes  are  4  feet  square  and  lined  with  2-inch  plank  placed  on  edge. 
When  the  sand  or  overlying  timber  appears,  a  new  slice  is  taken  off  the  pillar  and 
the  back  of  ore  is  caved,  as  before,  until  finally  all  of  the  subdrifts  have  been  worked 
out,  when  the  operation  of  caving  is  continued  in  the  block  below,  which  in  the 
meantime  will  have  been  honeycombed  by  the  first  or  preparatory  subdrifts. 

Below  the  eighth  level  the  method  of  mining  has  been  modified.  What  are 
called  intermediate  main  drifts  are  driven  through  the  ore  at  intervals  of  20  feet 

"Trans.  Am.  Inst.  Min.  Eng.,  Vol.  XXI,  1892,  p.  355. 


SOUDAN  FORMATION.  241 

instead  of  75  feet,  and  no  subdrifts  are  used.  The  intermediate  main  drifts  are  of 
the  regular  size,  9-foot  caps  and  7-foot  legs,  which  leaves  about  10  feet  of  ore  to  be 
caved,  instead  of  7  to  8,  as  before.  Stations  are  made  at  the  shaft  for  each  inter- 
mediate main  level.  Under  this  modified  system  the  removal  of  each  20-foot  block 
will  be  done  as  before,  but  the  putting  up  of  raises  will  be  saved,  and  it  is  intended 
to  use  cars  and  thus  do  away  with  wheelbarrows  as  far  as  possible. 

PI.  X,  B,  shows  a  main  drift  which  has  begun  to  cave  under  the 
weight  caused  by  the  wrecking  of  the  subdi-ifts  just  above.  PL  XI, 
shows  the  miners  removing  the  ore  from  a  part  of  the  mine  where  the 
caving  has  taken  place. 

The  ore  mined  \>j  the  Chandler  is  good  hard  hematite,  practicall}-  as 
hard  as  the  Soudan  ore.  Subsequent  to  its  formation  (p.  233)  it  was  frac- 
tured by  the  orogenic  forces  which  folded  the  rocks,  and  it  was  thereafter 
not  completely  cemented.  A  more  complete  healing  of  these  fractures 
seems  to  have  taken  place  in  the  ore  exposed  by  the  workings  of  the  Savoy 
•mine  (old  Section  26  mine),  at  the  eastern  end  of  the  Ely  trough.  The 
caving  system  described  above,  as  followed  in  the  Chandler  mine  under 
Manager  John  Pengilly,  takes  advantage  of  the  fracturing  which  already 
exists  in  the  ore  and  can-ies  it  farther  through  the  pressure  of  the  super- 
incumbent load.  As  a  result,  the  mass  of  ore  obtained  in  this  way  is  very 
much  brecciated,  so  that  it  can  readily  be  broken  up  by  picks.  In  conse- 
quence of  the  ease  with  whicli  it  can  be  obtained  by  picking,  the  ore  is 
frequently  erroneously  spoken  of  as  a  soft  ore.  The  individual  pieces  of 
the  breccia,  it  must  be  borne  in  mind,  are,  however,  the  hard  hematite, 
nearly  as  hard  as  that  of  the  Soudan  mines.  In  driving  some  of  the 
headings,  where  the  caving  has  not  affected  the  ore,  machine  drills  are 
very  frequently  employed. 

PRODUCTION  AND   SHIPMENTS  OF  IRON   ORE  FROM   THE  VERMILION  DISTRICT. 

The  following  tabulated  statement  gives  the  annual  production  and  ship- 
ments of  iron  ore  for  the  Vermilion  district  and  the  totals  for  the  district 
since  the  date  of  the  first  shipment  (1884)  up  to  the  present.  The  figures 
for  the  annual  production  during  the  early  years  of  the  mines  were  not 
obtainable,  but  have  been  given  as  a  lump  sum  for  those  years.  The  figures 
have  been  compiled  by  the  Minnesota  Iron  Company,  and  are  the  most 
accurate  that  can  be  obtained. 

MON  XLV — 03 16 


242  THE  VERMILION  IRON-BEARING  DISTRICT. 

Statement  showing  production  and  shipments  from  all  Vermilion  Range  mines  since  1881),. 


Minnesota  or  Soudan. 

Chandler. 

Production. 

Shipments. 

Production. 

Shipments. 

1884              1 

Long  tons. 

■  1, 182,  882 

312, 088 
553, 172 
518, 600 
513, 667 
568, 471 
429, 170 
448,  943 
412, 636 
427,  797 
502,  738 
426,  240 
441,  000 
310, 000 
257, 677 
307, 166 

Long  tons. 
62, 122 

Long  tons.  . 

Long  tons. 

1885 

227,075 

307,949 

.       394,911 

454,019 

1886 

1887                                                       ..           

1888                                                

169. 457 

54,  612 

1889                                

479,240  1         317.827 

306,  220 

1890                       

540, 013 
508, 842 
498, 353 
485,  778 
391, 612 
431,  647 
448, 970 
592, 244 
428, 054 
456,  225 
325,025 
208, 284 
275,168 

364,  659 
469,  741 
718, 014 
427,  595 
588,475 
347, 449 
551, 310 
586, 353 
628, 268 
648, 296 
644, 053 
659,  820 
593,  750 

336,  002 

1891            

373, 403 

1892 

651,  799 

1893 

435, 379 

562, 088 

1894 

1895 

600, 987 
471, 544 

1896 

1897                                            

438, 366 
716, 049 
808,  324 
644,  801 

1898 

1899                                         

1900                            

1901                   

627, 379 

1902                   -- 

645, 575 

Totals 

7, 612, 247 

7.515,531 

7,  715, 067 

7, 672, 528 

Year. 

Pioneer. 

Zenith. 

Savoy  and  Sibley. 

Production. 

Shipments. 

Production. 

Shipments. 

Production. 

Shipments. 

1884 

Long  tons. 

Long  ions.           Long  tons. 

Long  tons. 

Long  tons. 

Long  tons. 

1885 

........ 1-      

1886 

1887 

1888 

1889 

540, 299 

3, 144 

12,012 

3,079 

2,651 

1890 

1891 

1892 

I         97, 961 

r         14, 991 
14, 388 

1893 

1894 

1895 

40, 054 
149,  073 
207, 103 

' 

1896 

18,  765 
I        40, 817 

1897 

"29,408 

n  In  stock. 


SOUDAN  FORMATION. 


243 


Statement  showing  production  and  shipments  from  all  Vermilion  Range  mines  since 

18S4 — Continued. 


Pioneer. 

Zenith. 

Savoy  and  Sibley. 

Production. 

Shipments. 

Production. 

Shipments. 

Production. 

Shipments. 

1898 

Long  tons. 
30,  740 
381, 304 
492, 393 
620, 659 
669,  745 

Long  tons. 
123, 183 
339,  897 
450, 794 
678,  301 
673, 863 

Long  tons. 

3,924 

76, 303 

54, 252 

73, 512 

162, 006 

Long  tons. 

Long  tons. 

42 

97, 081 

175, 251 

194,  329 

324, 865 

Long  tons. 

1899 

79, 322 

60, 089 

60,  037 

167, 206 

86, 191 

1900 

175, 118 

1901 

211,  799 

1902 

321, 054 

Totals 

2,  735, 140 

2, 683, 154 

467, 9.58 

455, 615 

820, 976 

794, 162 

Long  tons. 

Total  production 19,351,-388 

'  Total  shipments 19, 120,990 

PROSPECTING. 

From  the  facts  of  occurrence  given  in  the  preceding  pages  we  are 
enabled  to  draw  the  following  conclusions  concerning  the  localities  at  which 
prospecting  for  ore  might  be  advantageously  prosecuted  in  the  Vermilion 
district  At  the  outset  it  may  be  said  that  as  a  result  of  the  mode  of  forma- 
tion and  occurrence  of  the  ores  it  is  very  probable  that  all  of  the  large 
ore  deposits  which  exist  will  somewhere  reach  the  top  of  the  iron  formation. 
However,  in  consequence  of  the  glacial  drift  which  covers  a  large  portion 
of  this  region,  the  ore  deposits  rarely  reach  the  present  surface  of  the 
ground,  being  buried  by  the  drift.  In  the  case  of  the  deposits  at  Tower 
and  Soudan,  the  ore,  on  account  of  its  exceptional  hardness,  outcropped  upon 
the  tops  of  the  highest  hills,  which,  owing  to  their  height,  are,  to  a  very 
considerable  extent,  free  from  the  drift.  But  usually  the  ore  is  softer  than 
the  adjacent  hard  jasper  and  chert  and  greenstones,  and  is  likely  to  have 
suffered  more  from  erosion  than  these.  Hence  the  ores  commonly  occu2)y 
more  or  less  marked  topographic  depressions. 

At  Ely  the  exposures  were  first  found  at  the  west  end  of  a  broad  basin, 
illustrating  what  the  writer  considers  the  tj^pical  occurrence,  at  least  for  the 
east  end  of  the  district.  Since  igneous  rocks  form  the  impervious  basements 
upon  whicli  the  ore  deposits  rest,  it  follows  that  the  igneous  rocks  should  be 
examined  with  great  care,  especially  where  they  are  impregnated  with  iron 
and  are  in  the  condition  in  which  they  are  known  as  the  paint  rock.    The  ideal 


244  THE  VERMILION  IRON-BEARING  DISTRICT. 

condition  for  the  accumulation  of  ore  is,  as  has  been  shown,  a  pitching  ti'ough 
of  greenstone,  with  a  great  thickness  of  fractured  jasper  lying  in  it.  The 
most  favorable  conditions  for  the  formation  of  an  ore  body  or  ore  bodies  of 
some  size  are  present  where  there  is  an  amphitheater  of  greenstone  within 
which  lies  the  jasper,  in  a  much  crumpled  and  fractured  condition,  with  the 
axes  of  the  synclines  plunging  toward  the  opening  of  the  amphitheater  of 
greenstone.  Unfortunately,  even  when  these  favorable  conditions  ai-e 
found,  only  exceptionally  has  the  accumulation  of  the  ore  taken  place. 

The  size  of  the  ore  bodies  varies  much  as  the  result  of  a  number  of 
factors,  but  one  can  state  with  confidence  that  the  larger  the  amphitheater 
and  the  larger  and  thicker  the  mass  of  iron  formation,  and  the  fewer  the 
dikes  contained  within  this  formation — whose  effect  would  be  to  subdivide 
the  large  pocket  into  a  number  of  smaller  ones — the  more  .likely  will  an 
ore  deposit  found  prove  to  be  a  large  one. 

The  Tower  and  Soudan  dejDOsits  are  in  synclines  which  occur  on  the 
top  of  anticlines,  foi'ming  hills.  As  the  result  of  this  known  occurrence, 
the  prospectors  have  appeared  very  generally  to  neglect  explorations  in  low 
ground.  But  there  are  a  number  of  areas  of  low  ground  that  are  with 
great  degree  of  probability  underlain  by  the  iron  formation,  and  some  of 
them  possibly  by  iron  ore,  which  should  be  explored,  for,  as  already  noted, 
where  the  ore  is  soft  it  is  generall}^  found  to  occupy  the  lower  areas.  The 
difficulties  attending  prospecting"  in  these  low  areas  are  great,  on  account  of 
the  water  and  the  deep  drift  frequently  found  in  them;  but  they  may 
contain  ore  deposits  which  will  pay  in  proportion  to  the  difficulties 
attendant  upon  their  discovery.  It  may  be  well  to  call  to  mind  the  fact 
that  some  of  the  large  deposits  occurring  in  Micliigan — for  instance,  the 
Aragon  mine  of  the  Menominee  range  and  the  Lake  Angeline  of  the 
Marquette  range — are  found  in  such  positions. 

But  it  should  be  emphasized  that,  as  a  matter  of  experience,  no  large 
ore  dei)0sits  have  been  found  except  where  the  iron-bearing  formation  has 
considerable  breadth.  At  many  points  the  favorable  conditions  mentioned 
above,  except  the  presence  of  broad  bands  of  the  iron-bearing  formation, 
have  been  found;  but  in  no  known  case  where  the  iron  formation  is  narrow 
have  such  localities  yielded  workable  ore  deposits.  Certainly  experience 
in  the  Vermilion  district  does  not  justify  the  expenditure  of  money  in 
exploring  the  narrow  bands  of  jasper.     Many  thousands,  probably  hundreds 


SOUDAN  FORMATION.  245 

of  thousands,  of  dollars  have  beeu  spent  in  exploring  these  narrow  bands 
without  any  returns. 

A  close  study  of  the  map  shows  some  places  whicli  seem  favorable  for 
prospecting.  In  a  general  survey  it  is  impossible  to  study  a  district  in  such 
detail  as  to  warrant  an  expression  of  opinion  as  to  individual  localities.  In 
fact,  such  A^'ery  detailed  study  with  a  view  of  determining  the  exact  location 
of  ore  deposits  can  hardly  be  considered  a  j^art  of  the  functions  of  a 
national  survev. 

-  The  exploration  of  the  iron-bearing  formation  as  an  economic  problem 
should  be  handled  by  the  mining  companies.  They  would  unquestionably 
find  it  greatl}'  to  their  advantage  to  have  competent  geologists  make  exceed- 
ingly detailed  surveys  of  properties  in  which  there  are  large  belts  of  the 
iron  formation.  If  such  surveys  show  that  certain  areas  have  conditions 
favorable  for  ore  deposits  it  would  be  advisable  to  burn  over  such  areas 
so  as  to  increase  the  number  (if  exposures,  and  thereby  make  the  areas  more 
accessible.  Finally  the  favorable  locations  should  be  narrowed  down  still 
more  by  careful  dip  and  horizontal  magnetic-needle  observations.  Only 
when  all  this  preliminary  work  is  done  should  a  decision  be  made  in  refer- 
ence to  underground  work. 

The  cost  of  such  preliminary  surveys  as  are  advocated  is  insignificant 
when  compared  with  the  money  required  for  diamond  drilling  and  other 
underground  work.  Many  such  expenditures  in  the  past  would  not  have 
been  made  had  the  results  of  the  surveys  advocated  been  available.  Apropos 
of  the  cost  of  the -diamond-drill  work,  it  may  be  stated  that,  from  informa- 
tion derived  from  various  sources,  the  estimate  has  been  made  that  one  of 
the  old  com^Dauies  operating  in  the  Vermilion  district  expended  at  least 
Si, 000, 000  in  exploratory  work  without  having  uncovered  thereby  any 
body  of  ore  of  size  sufficient  to  warrant  its  exploitation.  The  preliminary 
expenditure  of  a  very  small  fraction  of  this  amount  of  money  for  good  sur- 
veys in  advance  of  underground  work  would  have  made  unnecessary  the 
expenditure  of  much  of  it,  and  perhaps  would  have  rendered  the  expenditure 
of  a  part  fruitful. 

In  conclusion,  it  may  be  recalled  that  the  only  productive  mines  are  at 
two  localities,  one  on  the  belt  of  ir<m  formation  near  Tower  and  Soudan,  and 
the  other  on  the  belt  of  iron  formation  running  east  from  Ely.  Notwith- 
standing extensive  but  more  or  less  haphazard  exploration  of  other  large 


246  THE  VERMILION  IRON-BEARING  DISTRICT. 

belts  for  many  years,  no  additional  deposits  have  yet  been  developed. 
However,  it  is  by  no  means  proved  that  some  of  these  belts  may  not  yield 
runs  of  ore.  Bnt  the  iron-bearing  formation  and  the  Ely  greenstone  are 
so  intimately  mixed  that  some  of  the  belts  which  seem  to  be  largely  iron 
formation  may  be  really  to  a  very  large  extent  composed  of  greenstone  and 
jasper.  Under  these  conditions,  the  systematic  exploration  of  even  those 
belts  which  appear  to  be  the  most  promising-  is  a  matter  of  extraordinary 
difficulty.  Certainly  the  Vermilion  district  is  the  most  difficult  to  explore 
of  any  of  the  iron-bearing  districts  within  that  part  of  the  Lake  Superior 
region  in  the  United  States. 

-         .  SECTION  IV— GRANITES. 

GENERAI.,  STATEMENT. 

At  a  great  number  of  places  throughout  the  Vermilion  district  acid 
rocks  of  various  kinds  have  been  found.  Their  macroscopic  and  microscopic 
features  demonstrate  their  igneous  character  without  possibility  of  question, 
and  their  relations  to  the  adjacent  rocks  give  further  proof  of  this,  as  they 
are  found  cutting  through  both  the  Ely  greenstones  and  the  iron-bearing 
Soudan  formation  of  the  Archean.  These  rocks  vary  from  fine-  to  coarse- 
grained granites  and  from  porphyries  with  very  fine-grained  groundmass  to 
granite-porphyries.  The  normal  granites  predominate.  They  are  known 
from  the  topographic  featm-es  with  which  they  are  associated  as:  (1)  The 
granites  of  Vermilion  Lake;  (2)  the  granites  of  Trout,  Burntside,  and 
Basswood  lakes;  (3)  the  granite  between  Moose  Lake  and  Ivawishiwi 
River;  (4)  the  granite  of  Saganaga  Lake.  These  granites  will  be  considered 
in  detail  in  this  section. 

THE  AGE  OF  THE  ACID  INTRFSIVES. 

All  of  these  rocks  are  younger  than  the  Ely  greenstone,  for  they 
occur  in  it  as  dikes.  A  number  of  the  dikes  are  also  found  in  the  Soudan 
formation,  which  is  itself  of  more  recent  origin  than  the  greater  part  of  the 
Ely  greenstone.  That  these  intrusives  are  older  than  the  next  sedimentary 
formation  of  the  district — the  Ogishke  conglomerate,  of  Lower  Huronian 
age,  which  succeeds  the  Soudan  formation — is  shown  positively  b}- 
the  fact  that  thej^  occur  as  pebbles  in  this  conglomerate,  and  that  their 
detritus  largely  constitutes  the  rocks  of  this  formation.     Speaking  broadly, 


ARCHEAN  GRANITES.  247 

the  g-enei-al  period  of  intrusion  of  all  of  these  acid  igneous  rocks  may  be 
placed  between  the  period  of  the  deposition  of  the  latest  sediments  of  the 
Archean  and  that  of  the  deposition  of  the  earliest  sediments  of  the  Lower 
Huronian  series.  Some  were  intruded  near  the  beginning'  of  this  interval, 
others  probably  near  the  end,  but  it  is  now  impossible  to  give  their  exact 
ages.  In  the  description  of  the  rock  from  each  of  the  large  areas  after 
which  it  is  named  an  attempt  will  be  made,  where  there  are  any  facts  which 
warrant  this,  to  determine  more  closely  its  period  of  intrusion  relative  to 
the  other  igneous  rocks  as  well  as  to  the  sediments. 

GRAjSTITE  of  VERMILIOIS^  LAKE. 

DISTRIBUTION,  EXPOSURES,  AND  TOPOGRAPHY. 

Distribution.— Granites  and  related  acid  rocks  are  found  in  great 
quantity  on  the  islands  in  Vermilion  Lake  and  on  the  adjacent  shores. 
They  are  not,  however,  confined  in  their  distribution  to  the  immediate 
vicinity  of  the  lake,  for  scattered  dikes  of  similar  rocks  are  found  cutting 
tlu'ough  the  various  older  formations  in  places  many  miles  distant  from 
Vermilion  Lake.  Direct  surface  connection  of  these  distant  dikes  with 
the  main  masses  can  not  of  course  be  shown,  but  from  their  similarity  to 
the  larger  masses  in  the  area  it  is  presumed  that  both  are  derived  from  the 
same  deep-seated  source. 

JExposures. — The  exposures  of  these  acid  rocks  are  very  numerous,  and 
many  of  them  afford  opportunity  for  a  study  of  their  different  kinds,  but 
they  rarely  show  more  than  a  single  contact  when  the  contact  is  between 
members  of  the  same  eruptive  series,  so  that  in  most  cases  it  is  impossible 
to  tell  the  exact  relations  of  these  rocks  to  one  another — that  is,  to  determine 
which  is  of  younger  age.  Some  of  the  best  places  at  which  to  see  these 
Vermilion  Lake  intrusives  are  the  east  end  of  Ely  Island,  the  point  south 
of  Mud  Creek  Bay,  Stuntz  Island,  the  conical  island  east  of  Stuntz,  the 
prominent  point  of  land  farther  east  of  this  conical  island,  and  the  high, 
bare  hills  on  the  north  side  of  the  "Burnt  Forties."  Dikes  of  these  rocks 
are  also  numerous  in  the  Ely  gTeenstone  north  of  Mud  Creek  Bay,  where 
one  of  them  about  30  feet  wide,  cutting  the  greenstone  and  trending  east 
and  west,  can  readily  be  seen  from  the  water's  edge  as  a  white  streak  along 
the  hillside. 

On  the  flanks  of  knolls  occupied  by  the  igneous  rocks  we  very  commonly 


248  THE  VERMILION  IRON-BEARING  DISTRICT. 

find  sedimeiltary  rocks — eouglomerates  and  graywackes — whicli  have  been 
derived  from  tliem.  In  many  cases,  moreover,  it  is  only  after  very  careful 
and  patient  examination  that  the  igneous  rocks  can  be  separated  from  the 
derived  sedimentaries,  for  where  the  constituents  of  these  derivative  rocks 
have  been  merely  cemented  together  without  having  been  much  rolled 
and  rounded,  and  where  the  deposits  are  not  well  stratified,  the  resemblance 
between  the  true  igneous  rocks  and  the  rocks  derived  from  them  is  very 
great  indeed. 

The  igneous  rocks  that  occur  at  a  considerable  distance  from  the  lake 
are  exposed  over  only  very  small  areas.  In  some  cases  the  exposure  is  suf- 
ficient to  show  that  the  rocks  are  dikes  in  older  rocks,  and  very  commonly 
this  relation  is  infen-ed  from  the  occurrence  of  these  rocks  in  the  midst  of 
numerous  exposures  of  rocks  belonging  to  formations  which,  from  facts 
observed  in  other  localities,  are  known  to  be  of  greater  age  than  the 
eruptives. 

Topography. — The  eruptives  usually  occupy  the  crests  of  hills,  or  occur 
in  rounded  or  oval  hills  higher  than  those  occupied  by  the  surrounding- 
rocks.  They  thus  are  seen  to  influence  the  topography  to  a  considerable 
extent.  This  influence  is,  of  course,  best  shown  in  those  areas  where  the 
rocks  occur  in  large  quantity,  as,  for  example,  the  islands  in  Vermilion 
Lake  and  the  lake  shores,  rather  than  at  places  some  distance  away,  where 
they  occur  as  very  small  masses. 

PETROGRAPHIC  CHARACTERS. 

The  rocks  considered  in  this  section  form  a  complex  varying  in  both 
macroscopic  and  microscopic  characters,  as  well  as  in  chemical  composition, 
yet  in  spite  of  these  variations  their  close  field  relations  and  their  char- 
acters as  determined  by  laboratory  study  show  that  they  all  belong  to  one 
petrographic  province  and  that  they  were  formed  at  the  same  geologic 
period. 

All  of  the  rocks  belonging  to  this  series  of  eruptives  are  very  light 
colored,  at  the  most  showing  slatj^-gray  to  gTeenish-gray  colors  upon  fresh 
fracture.  On  weathered  surfaces  they  are  usually  white  or  light  gray, 
varying  to  yellowish  i>r  pinkish. 

Macroscopic  characters. — The  rocks  under  discussion  vary  from  fine- 
grained granites  to  those  of  coarse  grain,  and  from  porphyries  with  telsitic 
groundmass   and    rare  phenocrysts    to  coarse-grained    granite-porphyries. 


ARCHEAN  GRANITES.  249 

The  porphyritic  rocks  are  most  common.  Quartz  is  the  usual  phenocryst, 
but  it  is  sometimes  accompanied  by  feldspar.  In  some  of  the  porphyries 
the  phenocrysts  are  very  abundant,  but  in  others  they  are  very  scarce.  The 
quartz  phenocrysts  range  in  size  from  those  that  are  scarcely  separable  from 
the  quartz  of  the  groundmass  up  to  crystals  nearly  an  inch  in  diameter. 
Two  characteristic  porphyries  especially  are  of  common  occurrence  in  the 
district.  One  contains  a  great  number  of  small  vitreous-looking  quartz 
phenocrysts;  the  other  usually  shows  only  a  few  very  large  phenocrysts. 
The  quartz  phenocrysts  differ  greatly  in  number  in  different  types  of  the 
rock.  In  some  of  the  rocks  only  a  few  are  present;  in  others  they  occur 
in  great  abundance.  Moreover,  the  quartz  shows  considerable  variation  in 
character.  Usually  it  is  clear  and  vitreous;  less  commonly  it  has  a  some- 
what bluish  tinge,  but  is  still  vitreous.  Very  commonly  white,  opaque, 
porcelain-like  phenocrysts  appear,  and  some  are  found  that  are  black  and 
opaque.  Usually  the  quartz  phenocrysts  of  the  rock  occurring  at  a  single 
exposure  are  all  alike;  that  is,  all  are  clear  and  colorless  or  all  are  black, 
but  sometimes  these  are  found  intermingled. 

Feldspar  in  white  crystals  appears  also  as  phenocrysts  in  the 
porphyries  and,  like  the  quartz,  varies  much  in  abundance.  The  dark 
2^henocrysts  seen  in  these  rocks  are  either  mica  or  hornblende,  or 
occasionally  the  two  together,  but  as  a  rule  dark  phenocrysts  are  scarce. 

The  groundmass  of  these  porphyries  gives  to  them  some  of  their 
distinctive  characters.  In  some  the  groundmass  is  dense  and  aphanitic; 
in  others  it  is  distinctly  granular;  in  still  others  it  is  coarse  grained; 
moreover,  there  are  phases  of  the  groundmass  that  show  all  gradations 
between  these  different  kinds.  In  some  of  the  porphyries,  as  has  been 
said,  the  phenocrysts  are  practically  wanting,  and  as  these  become  reduced 
in  quantity  the  rocks  gradually  change  to  those  which  we  would  call 
granites;  and  we  find  not  only  changes  from  porphyries  into  granites, 
but  gradational  phases  among  the  granites,  varying-  from  fine-grained 
microgranites  to  normal  coarse-grained  granites. 

The  granites  and  porphyries  of  Vermilion  Lake  occasionally  include 
fragments  of  the  iron-bearing  Soudan  formation,  both  large  and  small,  as 
well  as  fragments  of  greenstones,  but  they  contain  no  inclusions  of  a 
recognizably  sedimentary  rock  other  than  those  derived  from  the 
iron-bearing  formation. 


250  THE  YERjVHLION  IRON-BEARING  DISTRICT. 

Microscopic  characters. — The  result  of  microscopic  examination  sustains 
tlie  determinations  made  by  macroscopic  studies.  It  shows  that  there  are 
present  in  the  acid  iutrusives  of  Yennilion  Lake  the  following  peti'Ographic 
varieties:  Rhyolite-poi-phyry,  feldspathic  porphyry,  microgranite,  granite, 
microgranite-porphyry,  and  granite-poi-]^)h}T.y.  The  minerals  occurring 
in  all  of  these  are  essentially  the  same.  Under  the  microscope  quartz  is 
the  most  prominent  primary  constituent,  and  ranges  from  minute  particles 
taking  part  in  the  construction  of  the  groundmass  up  to  phenocrysts  an  inch 
in  length.  Both  orthoclase  and  jjlagioclase  feldspar  occur  in  the  groundmass 
and  as  phenocrysts.  Polysynthetically  twinned  plagioclase  predominates 
among  the  phenocrysts.  These  feldspars  are  much  altered  and  an  accurate 
determination  of  their  characters  was  not  made.  Brown  mica  occurs  occa- 
sionally as  phenocrysts  and  is  almost  always  altered  to  chlorite.  A  few 
indi\dduals  of  common  green  hornblende  were  observed,  which  appear  to 
be  primarv.  Apatite,  sphene,  zircon,  and  a  little  iron  oxide  were  also 
observed.  From  these  various  minerals  there  have  been  produced  by 
alteration  the  following  secondary  minerals:  Calcite,  which  is  distinctly 
fen-iferous,  chlorite,  epidote,  zoisite,  sericite,  muscovite,  and  rutile.  Pyrite 
in  cubes  is  also  commonly  found  in  some  of  the  altered  intrusives.  The 
texture  of  these  rocks  is  normally  granitic,  although  occasionally  a  tend- 
ency to  a  trachytic  texture  was  observed  in  some  of  the  poi-phyries  and 
more  commonly  a  niicropegmatitic  texture  was  seen. 

No  analyses  of  these  inti'usive  rocks  have  been  obtained.  Indeed  no 
special  effort  has  been  made  to  obtain  analyses,  for  the  reason  that  their 
field  associations  and  general  characters  show  clearly  that  the  vai-ious  kinds 
of  rock  included  under  the  above  head  belong  genetically  together,  and 
for  the  further  reason  that  the  rocks  are  without  exception  considerably 
altered,  so  that  analyses  might  be  misleading  rather  than  helpful.  It  is 
highly  probable  that  analyses  of  fresh  rocks,  could  such  be  obtained,  would 
show  that  these  acid  intnisives  range  from  the  granites  toward  the  diorites 
by  increase  in  soda-lime-feldspar  and  diminution  in  quartz  and  orthoclase, 
as  is  indicated  in  some  of  the  feldspathic  porphyries. 

FOLDING. 

These  intrusives  have  been  subjected  to  dynamic  action,  for  they  are 
very  commonly  jointed,  and  in  some  places  even  rendered  schistose.  They 
have  also  taken  part  in  the  folding,  but  owing  to  their  general  homogeneous 


ARCHEAN  GRANITES.  251 

nature  it  is  impossible  on  exposures  consisting  of  intrusive  rocks  alone  to 
trace  out  the  extent  and  character  of  this  folding-.  Where  the  intrusives 
are  associated  with  younger  sedimentary  rocks  the  folding  is  clearly  shown, 
and  is  described  on  pages  288-291. 

STRUCTURAL  FEATURES  AND   METAMORPHISM. 

As  a  rule  the  intrusives  are  broken  up  by  a  series  of  joints,  which  are 
sometimes  so  close  together  that  the  rocks  l)reak  up  into  small  more  or  less 
regular  rhombs.  The  very  general  distribution  of  the  jointing  seems  to 
point  to  the  fact  that  the  rocks  were  not  buried  very  deep,  for  had  they 
been  so  buried  it  is  probable  that  they  would  have  acted  more  as  viscous 
materials,  and  that  schistosity  would  have  been  developed,  instead  of  the 
stress  being  relieved  hj  the  formation  of  joints,  as  is  the  case.  As  a  rule, 
however,  the  rocks  composing  the  blocks  between  the  fractures  show  no  or 
very  slight  indications  of  schistosity.  On  these  jointed  rocks  it  is  not 
uncommon  to  find  a  few  shearing  planes  along  which  schistosity  has  been 
developed.  Moreover,  the  rock  is  sometimes  schistose  along  the  small 
fracture  planes  themselves.  Reference  has  casually  been  made  to  the 
difficulty  sometimes  experienced,  even  under  favorable  circumstances,  of 
distinguishing  between  these  igneous  intrusives  and  some  of  the  sedimentary 
rocks  derived  directly  from  them.  When  schistosity  has  been  developed 
in  the  igneous  rock,  even  though  it  be  imperfectly  developed,  the  difficulty 
is  much  increased,  whether  the  test  applied  be  that  of  macroscopic  or 
microscopic  examination,  or,  as  in  most  cases,  the  two  combined. 

Some  very  interesting  occuiTences  of  pseudo-conglomeratic  rocks 
derived  from  these  acid  rocks  by  orogenic  movement  may  be  seen  at 
Vermilioii  Lake.  One  of  the  best  cases  is  shown  on  the  flat  island  lying 
just  south  of  Ely  Island,  in  the  NE.  i  of  the  NE.  i  of  sec.  30,  T.  62  N., 
R.  15  W.  This  island  is  composed  chiefly  of  rhyolite-porphyry,  with  fine 
felsitic  groundmass,  having  large  quartz  phenocrysts  scattered  through  it. 
This  porphyry  is  intersected  by  two  sets  of  partings,  which  vary  from  big 
joints  down  to  very  minute  partitig  planes.  One  of  these  sets  of  fractures 
strikes  N.  16°  E.  and  the  other  N.  80°  W.  These  planes  of  fracture 
separate  the  rock  into  numberless  more  or  less  regular  rhombs.  The  large 
joints,  a  fo.ot  or  more  distant  from  one  another,  break  the  rock  into  large 
rhombs,  which  in  their  turn  are  subdivided  into  a  great  number  of  smaller 


252  THE  VERMILION  IRON-BEARING  DISTRICT. 

ones  by  systems  of  still  smaller  joints  and  parting  planes  lia\'ing  the  same 
trend  as  the  larger  fractures.  After  the  formation  of  the  joints  movement 
took  place  along  the  fracture  planes,  and  as  a  result  of  this  movement  the 
i-hombs  have  been  rubbed  against  one  another  and  their  angles  have  been 
more  or  less  completely  rounded,  so  that  the  rhombs  have  acquired  now  a 
more  or  less  perfect  oval  outline.  The  long  axes  of  the  ovals  agree,  of 
course,  since  the  ovals  were  all  produced  by  the  same  processes.  The 
rock  is  now  strikingly  like  a  conglomerate,  and  forms  a  fine  example 
of  a  pseudo-conglomerate.  This  pseudo-conglomerate  may,  nevertheless, 
readily  be  distinguished  from  true  conglomerates,  such  as  occur  in  abundant 
typical  development  at  Vermilion  Lake,  and  especially  on  the  shores  of 
Stuntz  Bay,  by  the  fact  that  all  of  the  pebbles  of  the  pseudo-conglomerate 
are  of  exactly  the  same  kind  of  porphyry,  and  that  the  matrix  between 
the  pebbles  is  merely  a  sheared  form  of  the  same  porphyry.  Moreover, 
no  indication  of  bedding  whatsoever  is  found  in  this  pseudo-conglomerate. 
The  true  conglomerates  contain  pebbles  of  various  porphyries,  as  well  as 
of  the  older  greenstone  and  the  iron-bearing  formation,  and  gradations  can 
be  followed  from  these  conglomerates  into  the  graywackes,  and  through 
these  into  the  overlying  slates.  The  pseudo-conglomerate  is  most  typically 
developed  at  the  location  given  abpve.  It  is  seen  in  less  typical  develop- 
ment on  the  point  south  of  Mud  Creek  Bay  and  at  some  other  localities  on 
Vermilion  Lake.  It  is  not  so  common,  however,  as  one  might  be  led  to 
suppose  fi'om  previous  descriptions  of  the  Vermilion  district."  Far  more 
numerous  are  the  exposures  of  the  porphyi')'  on  which  the  fracturing  is  not 
very  distinct,  and  on  which  movement  has  not  been  sufficient  to  produce 
the  rounding  of  the  rhombs  and  the  pseudo-conglomeratic  structure.  These 
pseudo-conglomerates  were  desci'ibed  by  Smyth  and  Finlay'  under  the 
name  "conglomerate  breccias,"  and  the  manner  in  wliich  tliey  were  formed 
was  correctly  interpreted.  However,  these  authors  vinfortnnateh'  classed 
in  their  conglomerate  breccias  the  enormously  and  typically  developed 
sedimentary  conglomerates  that  occur  on  the  islands  and  shores  at  the  east 
and  southeast  side  of  Vermilion  Lake,  and  in  particular  on  Stuntz  Bay  of 
that  lake,  where  they  are  interbedded  with  and  grade  into  the  normal 
fiuer-gi'ained  graywackes  and  slates. 


"Thej.'i'olngiciilfitnK'tiiivof  the  wes^tern  part  of  the  Vermilion  Range,  ^linnesota,  by  11.  L.  Smyth, 
and  .J.  Ralph  Finlay:  Tran.s.  Am.  Inst.  Min.  Eng.,  Vol.  XXV,  1895,  pp.  610-613. 
''Op.  cit.,  pp.  629-6,33. 


ARCHEAN  GRANITES.  253 

Sericite-schists. — When  the  crushing-  and  accompanying-  alteration  are 
considerably  advanced,  sericite-schists  are  prodviced  from  these  porphyries 
and  granites.  At  one  stage  we  find  a  few  eyes  of  quartz  and  feldspar  left. 
These  lie  in  a  finely  granular  groundmass  in  which  the  parallel  structure  of 
the  secondary  minerals  is  very  evident.  This  parallelism  is  most  pro- 
nounced when  the  rock  contains  a  g-reat  deal  of  sericite,  for  then  the  plates 
of  sericite  are  arranged  parallel  to  one  another  and  greatly  emphasize  the 
structure.  The  parallel  arrangement  follows  around  the  ^^henocrysts, 
showing  that  it  was  produced  after  their  foi'mation.  It  may  be  that  in 
some  cases  this  parallelism  represents  partly  an  original  flow  structure  which 
has  been  emphasized  by  the  production  of  the  secondary  minerals.  The 
extreme  stage  shows  a  very  fine-g'rained  schistose  rock  which  is  of  a 
yellowish-green  color  macroscopically,  and  which  under  the  microscope  is 
seen  to  be  a  finely  granular  aggregate  of  quartz,  presumably  some  feldspar, 
and  flakes  of  sericite,  these  being  the  predominant  minerals. 

CMorite-scMsts. — In  a  number  of  cases  the  porphyries  show  abnormal 
alteration  to  a  green  chlorite-schist  instead  of  to  a  sericite-schist.  Such 
alteration  was  found  in  immediate  association  with  the  iron-bearing  forma- 
tion— that  is,  where  the  acid  rocks  had  been  intruded  and  then  infolded  in 
the  iron  formation.  The  production  of  the  chloi'ite  and  of  the  green  color  in 
g-eneral  is  due  to  the  infiltration  of  iron  from  adjacent  foi'mations.  In  some 
instances  the  green  coloration  is  found  only  along  the  contact  of  an  acid 
dike  with  the  iron  formation  and  extends  only  a  few  inches  into  the 
porphyry.  In  other  cases,  where  the  acid  rock  is  in  the  midst  of  the 
iron  formation,  the  rock  is  distinctly  green,  and  might  be,  and  in  fact  has 
been  taken  for  a  product  derived  from  the  altered  basic  rocks  of  the  area — 
the  greenstones.  In  some  places  the  quartz  phenocrysts  have  been  granu- 
lated, but  in  others  they  are  still  intact  and  show  clearly  the  fact  that 
these  schists  were  derived  from  acid  rocks.  The  efi^ect  of  the  iron  in 
causing  the  production  of  chlorite  instead  of  sericite  can  be  seen  in  many 
places  in  the  massive  acid  rocks.  In  these  we  very  commonly  find  that 
when  iron  pyrites  occurs  it  is  almost  invariably  surrounded  by  a  zone  of 
limonite  of  variable  thickness,  and  beyond  this  zone  of  limonite  there  is 
a  chloritic  zone  in  the  groundmass,  whereas  elsewhere  sericite  occurs  and 
not  chlorite. 

Schistose  granites  and  schists  derived  from  granites. — Locally  these  gran- 
ites have  been  very  much  crushed,  and  as  a  result  of  this  crushing  there 


254  THE  VERMILION  IRON-BEARING  DISTRICT. 

has  been  developed  in  a  number  of  cases  a  parallelism  of  the  feldspar  and 
quartz  and,  especially,  of  the  mica  and  secondary  chlorite.  The  qu^.rtz 
shows  clearly  the  effects  of  the  crushing  in  the  very  common  undulatory 
extinction,  in  the  fractures  that  pierce  the  phenocrysts,  and  in  the  granula- 
tion of  the  phenocrysts,  which  represents  the  final  stage.  This  crushing 
has,  of  course,  more  or  less  completely  obliterated  the  textures  and  has 
usually  greatly  altered  the  minerals.  Fractures  passing  through  rocks  have 
been  healed  by  infiltrated  quartz,  or  by  secondary  feldspar  in  cases  where 
the  fractures  cross  the  feldspar  phenocrysts.  In  such  a  case  the  secondary 
feldspar  corresponds  in  extinction  with  the  adjacent  feldspar  bordering  the 
fracture,  but  is  fresh  and  clear,  and  is  readily  distinguishable  by  these 
characters  from  the  altered  original  feldspar. 

RELATIONS  TO  ADJACENT  FORMATIONS. 

The  acid  rocks  described  above  are  younger  than  the  adjacent  Ely 
greenstones  and  Soudan  formation.  They  occur  in  dikes  in  both  of  these 
formations  and  include  fragments  of  both.  Detailed  descriptions  of  the 
occurrences  of  some  of  these  rocks  will  be  found  under  the  heading  "Inter- 
esting localities  "  (p.  255). 

Relations  to  Lower  Huronian  series. — The  relations  of  the  granite  of 
Vermilion  Lake  to  the  Lower  Huronian  sediments  will  be  discussed  in 
detail  later.  It  will  suffice  here  to  state  that  these  sediments  have  been 
derived  partly  from  the  acid  rocks  and  hence  are  younger  than  they.  The 
detailed  proof  of  their  relations  will  be  given  in  the  chapter  devoted  to  the 
discussion  of  these  sediments. 

Interrelation  of  granites  of  Vermilion  Lake. — The  acid  intrusives,  while 
of  the  same  general  age  with  respect  to  the  older  and  younger  sedimentary 
formations,  show  certain  age  relations  among  themselves  which  are  inter- 
esting. The  fine  granite  seems  to  be  rather  more  extensively  developed, 
on  the  whole,  than  the  rhyolite-porphyries  and  the  granite-porphyries. 
This  granite  is  cut  at  several  points  by  dikes  of  fine-grained  granite-por- 
phyr)^  containing  small  quartz  phen<x',rysts — for  instance,  on  the  point 
south  of  Mud  Creek  Bay,  on  Stuntz  Island,  on  the  island  just  west  of 
Stuntz  Island,  and  at  a  locality  just  north  of  the  prominent  jasper  outcrop 
on  the  east  side  of  Stuntz  Bay.  This  granite-porphyry  is  in  its  turn  found 
to  be  intruded  by  the  granite-porphyry  containing  the  large  quartz  eyes. 


ARCHEAN  GRANITES.  255 

Such  an  occurrence  was  observed  in  the  Burnt  Forties,  for  example.  Thus 
the  succession,  beginning  with  the  oldest  of  the  intrusives,  is:  Fine-  to 
medium-grained  granite,  fine-grained  granite-porphyry  with  small  eyes, 
and  coarse  granite-porphyry  with  large  quartz  eyes.  The  relation  of  the 
rhyolite-porphyrj-  to  the  other  acid  eruptives  is  not  definitely  known,  as  no 
occurrence  has  been  found  in  which  the  relations  between  them  are  shown. 

INTERESTING  LOCALITIES. 

Localities  shoiving  relation  between  granite  of  Vermilion  Lake  and  the  Ely 
greenstone. — The  relations  between  the  acid  intrusives  and  the  Ely  green- 
stone are  clearly  shown  at  many  places  in  the  Tower  area  of  the  Vermilion 
district.  In  the  following  paragraphs  some  of  the  most  accessible  of  these 
places  will  be  mentioned. 

On  the  north  shore  of  Mud  Creek  Bay,  in  sec.  1,  T.  62  N.,  R.  15  W., 
immediately  north  of  the  westernmost  island  in  this  bay,  there  is  a  broad 
dike  of  nearly  white  medium-grained  porphyry  which  trends  east  and  west, 
and  can  be  distinctly  seen  from  the  water.  This  dike  cuts  directly  across 
the  Ely  greenstone,  showing  sharp  contacts  with  it  in  many  places. 

Just  northeast  of  this  place,  on  the  section  line  between  sec.  1,  T.  62  N., 
R.  16  W.,  and  sec.  6,  T.  62  N.,  R.  14  W.,  are  dikes  of  granite  cutting  the  green- 
stone and  the  iron  formation  infolded  in  the  greenstone.  In  fact,  one  can 
hardly  go  a  quarter  of  a  mile  in  any  direction  over  these  nearly  bare  hills 
without  finding  one  or  more  of  these  acid  dikes.  The  greenstone  is  in  many 
places  schistose,  and  the  dikes  are  also  frequently  found  to  be  more  or  less 
schistose  along  their  margins,  the  schistosity  striking  a  little  north  of  west. 
The  presence  of  this  schistosity  is  clear  proof  that  the  area  has  been  folded 
subsequent  to  the  period  of  the  intrusion  of  these  igneous  rocks. 

About  the  center  of  sec.  6,  T.  62  N.,  R.  14  W.,  there  is  a  large  boss 
of  porphyritic  granite,  which  is  completely  surrounded  by  more  or  less 
schistose  gTcenstone.  Numerous  dikes  of  granite,  ranging  from  a  few 
inches  up  to  15  feet  in  width,  and  perfectly  massive,  evidently  ofi'shoots 
from  this  central  boss,  penetrate  the  greenstones.  Frequently  they  follow 
the  schistosity,  but  in  some  cases  they  cut  across  the  schistosity,  and  in 
places  they  include  fragments  of  the  schist.  Since  the  position  of  these 
intrusives  has  evidently  been  influenced  by  the  preexisting  schistosity,  they 
were  evidently    intruded    somewhat   after  those  that   have  already  been 


256  THE  VERMILION  IRON-BEARING  DISTRICT. 

mentioned.  They  are  believed,  however,  to  belong  to  the  same  general 
period  of  intrusion. 

On  the  hills  south  of  Mud  Creek  Bay  and  south  of  Mud  Creek  itself 
there  are  also  numbers  of  dikes  of  granite  and  porphyry  which  cut  the  ellip- 
soidal g-reenstone. 

The  following  are  other  localities  where  the  acid  intrusives  cutting  the 
greenstone  may  be  studied: 

North  1,000  paces,  west  1,000  paces,  from  southeast  comer  of  sec.  9, 
T.  62  N.,  R.  14  W.  Here  a  granite-porphyry  containing  large  phenocrysts 
of  quartz  cuts  through  a  dense  greenstone  forming  the  bluff  overlooking  the 
swamp  to  the  south.  A  similar  dike  is  to  be  found  at  north  1,650  paces, 
west  950  paces,  from  southeast  corner  of  sec.  21,  T.  62  N.,  R.  14  W. 

West  1,000  paces,  from  southeast  corner  of  sec.  10,  T.  62  N.,  R.  14  W. 
Here  the  porphyry  is  fine  grained,  and  has  a  dense  groundmass. 

North  200  paces,  west  100  paces,  from  southeast  corner  of  sec.  27,  T.  62 
N.,  R.  14  W.     This  is  one  of  the  feldspathic  porphyries. 

Belations  of  the  acid  intrusives  to  the  Soudan  formation. — On  the  bold,  bare 
hills  of  jasper,  at  a  point  north  270  paces,  west  200  paces,  from  the  south- 
east corner  of  sec.  7,  T.  62  N.,  R.  15  W.,  a  dike  of  granite-porphyry  20  paces 
wide,  containing  large  phenocrysts  of  quartz,  cuts  through  the  jasper.  It 
cuts  across  the  strike  of  the  bands  of  jasper  in  places  and  runs  out  into  the 
jasper  in  small  stringers,  and  also  includes  fragments  of  the  jasper.  The 
grain  of  the  intrusive  rock  is  seen  to  be  noticeably  finer  along  the  contact 
of  the  small  stringers  than  it  is  in  the  main  mass  of  the  granite.  Reference 
has  already  been  made  to  the  granite  dikes  found  cutting  the  jasper  in  sec. 
1,  T.  62  N.,  R.  15  W.  In  both  of  these  places  the  relations  are  perfectly 
clear. 

Contacts  between  these  two  kinds  of  rock  were  not  found  at  many 
places,  but  where  they  were  observed  the  relationship  was  clearly  shown. 
At  1,125  paces  north,  1,300  paces  west,  of  the  southeast  corner  of  sec.  20, 
T.  62  N.,  R.  14  W.,  a  dike  of  feldspar  porphyry  cuts  through  the  jasper  and 
the  associated  green  schist.  This  porphyry  includes  large  fragments  of  the 
jasper  and  small  ones  of  the  green  schist,  showing  conclusively  its  relations 
to  them.  In  places  these  fragments  are  so  numerous  that  the  rock  distantly 
resembles  a  conglomerate. 


ARCHEAN  GRANITES.  257 

Relations  of  the  different  varieties  of  the  acid  intrusives  of  Vermilion  Lake 
to  one  another. — On  the  bare  ridge  south  of  Mud  Creek  Bay  the  granite- 
porphyry  is  found  cutting  the  feldspathic  porphyry  at  several  places.  One 
such  dike  may  be  found  about  north  700  paces,  west  1,400  paces,  from  the 
southeast  corner  of  sec.  7,  T.  62  N.,  R.  14  W.  Again,  on  the  high  hill  in 
the  Burnt  Forties  overlooking  the  lake  the  granite-porphyry  is  found  in 
contact  with,  and  apparently  cutting,  the  fine-grained  porphyritic  granite, 
and  includes  fragments  of  greenstone  and  jasper.  It  is  intei-esting  to 
note  that  immediately  around  these  jasper  inclusions  the  acid  intrusive  has 
become  g-reen  as  the  result  of  the  infiltration  of  iron  and  the  production  of 
secondary  chlorite  instead  of  sericite.  From  this  green  background  the 
phenocrysts  of  quartz  stand  out  very  prominently.  A  similar  alteration 
occurs  in  the  acid  sills  that  were  intruded  through  the  iron  formation  on 
Soudan  Hill. 

East  of  Stuntz  Island  there  is  a  conical  island  which  is  made  up  chiefly 
of  the  fine-grained  feldspathic  poi'phyry,  and  on  which  there  is  a  dike  of 
porphyritic  granite,  about  25  paces  in  width,  running  from  northwest  to 
southeast.  At  certain  places  in  this  dike,  especially  on  the  southeast  slope 
of  the  island,  the  granite  grades  into  a  rock  corresponding  vexy  closely  to 
the  coarse-grained  granite-porphyry.  On  the  other  hand,  at  other  locali- 
ties, the  same  porphyritic  granite  was  observed  to  pass  into  a  form  of  rock 
very  similar  to  'Some  of  the  phases  of  the  feldspathic  porphyry.  It  would 
appear  from  this  that  these  different  kinds  of  rock  were  all  derived  from 
the  same  source,  and  that  they  merely  represent  different  phases  of 
development  of  essentially  the  same  magma.  Upon  this  conical  island 
there  were  noted  also  several  small  dikes  of  green  schistose  basic  rock,  one 
of  them  running  nearly  east  and  west  and  having  a  width  of  from  8  to  12 
inches. 

On  Stuntz  Island  itself,  especially  upon  the  northwest  arm  of  the 
island,  a  mass  of  this  porphyritic  granite  was  found  trending  about  east 
and  west,  cutting  through  the  feldspathic  porphyry. 

Nearly  all  of  these  porphyries  contain  more  or  less  oval  vellowish- 
green  fragments  of  rock  which  apparently  were  derived  from  the  greenstone 
through  which  they  were  intruded. 
MOX  XLV — 03 17 


258  THE  VERMILION  IRON-BEARING  DISTRICT. 

GBAIS^ITES    OF    TROUT,    BURIS^TSIDE,   AND    BASSWOOD    XiAKES. 

DISTRIBUTION,  EXPOSURES,   AND  TOPOGRAPHY. 

Distribution. — The  rocks  described  under  the  above  heading  occur 
along  the  northern  edge  of  the  Yermihon  district,  and  extend  from  Verniiliou 
Lake  on  the  west  to  the  east  side  of  Basswood  Lake  on  the  east,  where  they 
cross  the  international  boundary.  Observations  made  on  Hunters  Island, 
Province  of  Ontario,  Canada,  show  that  similar  rocks  are  present  in  that 
province  and  that  they  possess  the  same  geographic  and  geologic  relation- 
ship to  the  members  of  the  Kaministiquia  iron  range,  which  is  the  continua- 
tion of  the  Vermilion  iron  range  of  Minnesota  to  the  northeast,  as  do  their 
Minnesota  analogues  to  the  various  members  of  the  Vermilion  iron  range  of 
Minnesota.  They  are  for  this  reason  presumed  to  belong  geologically  with 
the  granite  of  Basswood  Lake.  While  it  is  known  that  the  granites  of 
Trout,  Burntside,  and  Basswood  lakes  extend  at  least  as  far  northeast- 
southwest  as  the  limits  of  the  area  mapped,  these  granites — or  granite 
closely  related  to  them — are  presumed  to  have  a  very  much  greater  areal 
extent,  as  upon  the  Canadian  maps  granites  are  shown  covering  large  areas 
in  portions  of  Ontario  which  are  continuous  with  the  Vermilion  range. 
The  Minnesota  and  Canadian  maps  and  a  traverse  luade  by  canoe  show 
that  ffranite  extends  a  considerable  distance  north  of  the  international 
boundary.  Since  the  prime  object  of  the  survey  whose  results  are  here 
described  was  to  study  the  Vermilion  di-strict  from  an  economic  point  of 
view,  no  attempt  has  been  made  to  study  the  outlying  granite  more  closely 
than  was  requisite  to  determine  its  relations  to  the  rocks  of  the  district. 
Moi'eover,  observations  have  been  confined  almost  altogether  to  a  very 
naiTow  area  bordering  the  main  mass  of  granite.  The  traverses  usually 
ended  as  soon  as  we  were  sure  that  the  limits  of  the  granite  had  been 
passed. 

Exposures. — The  exposures  are,  as  a  rule,  very  numerous,  and  the  line 
of  contact  between  the  main  granite  area  and  the  area  of  the  Vermilion 
range  proper  is  usually  marked  by  a  topographic  break  of  some  kind. 
Either  a  valley  and  stream  are  present  or  else  a  lake  or  chain  of  lakes  lies 
along  the  contact.  In  either  case  as  soon  as  the  depression  is  crossed,  if 
one  comes  from  the  south,  for  instance,  the  granite  exposures  usually  begin, 
and  they  continue  in  great  number  as  far  north  as  we  have  been. 


ARCHEAN  GRANITES.  259 

Topography. — The  grauite  does  not  seem  to  affect  tlie  topography  very 
materially.  One  point  noted  is  that  the  lakes  iu  the  area  of  the  sediments 
have,  as  a  rule,  a  northeast-southwest  trend,  agreeing  thus  with  the  struc- 
ture of  the  district,  upon  which  they  are  largely  dependent,  whereas  in  the 
granite  area  they  ai'e  more  likely  to  be  of  very  irregular  or  more  or  less 
rounded  outliiie,  owing  to  the  more  homogeneous  character  of  the  granites 
by  which  they  are  surrounded.  The  hills  in  the  granite  area  are  usually 
rounded  as  a  result  of  river  erosion  and  subsequent  glacial  action.  In 
detail  the  topography  is  very  rough,  as  is  that  of  all  this  portion  of  the 
country,  but  there  are  no  very  great  differences  in  elevation.  The  district 
underlain  by  the  granite  does  not  in  general  seem  to  have  been  much  more 
strongly  affected  by  erosion  than  the  adjacent  portions  of  the  Vermilion 
district.  In  the  course  of  a  reconnaissance  it  was  observed  that  on  the 
southeast  side  of  Iron  Lake,  which  is  on  the  international  boundary,  just  west 
of  Crooked  Lake,  there  is  an  area  of  country  that  has  been  reduced  almost 
to  a  base-leveled  plain,  with  Iron  Lake  as  the  plane  of  base-level.  The 
shores  of  the  lake  for  a  considerable  distance  back  from  the  water's  edge 
possess  all  the  features  of  such  a  base-leveled  plain.  The  streams  enter  the 
lake  through  broad  marshes  having  wide  estuaries  and  flow  in  meandering 
courses  through  these  marshes.  An  occasional  hill  (monadnock)  of  granite 
projects  above  this  level  plain.  Some  of  the  islands  in  the  lake  and  points 
projecting  into  it  are  so  low  that  in  many  places  by  rising  in  the  canoe  one 
can  see  over  them. 

PETROGRAPHIC  CHARACTERS. 

Macroscopic  characters. — The  granite  of  Trout,  Burntside,  and  Basswood 
lakes  shows  a  considerable  variation  in  character,  as  one  might  be  led  to 
expect  from  its  great  areal  distribvition.  In  color  it  varies  from  A'ery  light 
gray  through  pink  and  reddish  facies  to  very  dark  gray.  An  equal  variation 
in  grain  may  be  seen.  It  ranges  from  very  tine-grained  to  coarse-grained 
forms  and  also  to  granite-porphyries.  The  structure  of  the  rock  is  in  gen- 
eral massive,  but  with  these  massive  forms  occur  gneissoid  rocks  varying 
in  color  from  light  gray  to  very  dark.  Some  of  these  gneissoid  rocks  pre- 
sumably owe  their  structure  to  pressure  applied  subsequent  to  their  consoli- 
dation. In  these  the  minerals  show  to  a  large  degree  the  effects  of  pressure. 
Other  facies  may  be  due   to  differentiation  processes  and  to  movements  in 


260  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  unconsolidated  magma.  Between  rocks  formed  in  this  way  and  those 
formed  as  the  result  of  pressure,  but  in  which  complete  recrystallization  has 
taken  place,  no  distinction  can  be  made.  In  the  areas  examined  the 
massive  granites  predominate  greatly  over  the  schistose  rocks.  In  the  fol- 
lowing brief  description  only  the  massive  granites  will  be  considei'ed. 

Microscopic  characters. — The  mineral  constituents  are  green  hornblende, 
biotite,  orthoclase,  quartz,  and  plag'ioclase,  with  accessory  sphene,  zircon, 
and  iron  oxide.  These  minerals  have  been  very  much  altered,  so  that  their 
places  are  taken  largely  by  secondary  minerals,  of  which  chlorite  is  the 
most  prominent,  and,  after  this,  epidote  and  sericite  and  secondar)"  feldspar. 
There  is  a  variation  in  the  mineral  character,  hornblende  being  practically 
wanting  in  some  specimens  and  increasing  very  much  in  quantity  in  others. 
No  cases  were  found  in  which  the  quartz  was  wanting,  but  it  was  reduced 
in  quantity  in  some  cases.  The  Trout,  Basswood,  and  Burntside  lakes  acid 
rocks  seem  to  vary  from  hornblende-  and  mica-granites  to  syenites,  with  the 
granites  predominant. 

RELATIONS   TO   ADJACENT  FORMATIONS. 

Relations  to  Ely  greenstone. — In  approaching  that  portion  of  the  district 
in  which  the  granite  of  Trout,  Burntside,  and  Basswood  lakes  is  exposed  we 
cross  over  a  broad  ai'ea  underlain  by  the  Ely  gi-eenstone  in  its  typical 
development,  in  which  only  rarely  is  a  granite  dike  to  be  seen.  The  closer 
we  get  to  the  contact  between  the  two  above-mentioned  formations  the  more 
numerous,  however,  become  these  granite  dikes,  until  in  places  they  are  so 
common  that  we  may  almost  consider  the  greenstone  as  having-  been  thor- 
oughly permeated  by  the  granite  magma.  This  intimate  relationship  is 
beautifully  shown  on  the  numerous  exposiires  at  the  west  end  of  Burntside 
Lake.  It  should  be  stated  in  this  connection  that  the  granite  dikes  cutting 
through  the  rocks  exposed  on  the  shores  of  this  lake  do  not  all  belong  to 
exactly  the  same  period  of  intrusion,  but  show  some  slight  differences  in 
•  age.  These  differences  are  not,  however,  thought  to  be  great.  In  other 
words,  all  of  the  granites  are  believed  to  belong  to  essentially  the  same 
period  of  intrusion. 

From  the  intrusiA^e  relations  above  illustrated  it  is  clear  that  tlie  granite 
is  younger  than  the  adjacent  greenstone.  This  intrusive  relation  is  further 
emphasized  by  the  progressive  metamorphism  shown  by  the  Ely  greenstone 


ARCHEAN  GRANITES.  261 

as  exposures  closer  and  closer  to  the  main  granite  mass  are  examined,  as 
described  and  explained  on  p.  156  et  seq.  It  is  not  uncommonly  found  that 
the  greenstone  is  schistose,  and  the  granite  dikes  are  seen  to  follow  the 
schistosity,  proving  its  development  prior  to  the  intrusion  of  the  granite. 
The  granite  also  includes  fragments  of  schistose  greenstone. 

Relations  to  other  intrusive  rocks. — As  stated  above,  the  granite  areas 
were  not  studied  in  great  detail,  but  a  sufficient  number  of  observations 
were  made  to  show  that  the  granite  is  cut  by  both  acid  and  basic  dikes. 
The  basic  rocks  are  cut  by  acid  dikes,  as  is  shown  in  the  photograph  repro- 
duced on  PI.  XIII,  B.  The  normal  white  to  gray  granite  has  been  cut  by  a 
red-weathering  granite  which  traverses  it  in  dikes,  but  the  period  of  intru- 
sion of  these  later  red  granites  has  not  been  determined,  even  approximately. 

AGE. 

The  granite  of  Trout,  Burntside,  and  Basswood  lakes  is  evidently 
younger  than  the  Ely  greenstone,  which  it  cuts,  includes,  and  metamor- 
phoses. Dikes,  offshoots  from  it,  are  found  following  the  schistosity  of  the 
greenstone  and  including  these  schists.  Hence  it  was  certainly  intruded 
subsequent  to  the  formation  of  the  schistosity  in  the  greenstone.  This 
schistosity  was  prodiiced  primarily  as  a  result  of  the  folding  which  took 
place  subsequent  to  the  deposition  of  the  iron  formation  and  which  caused 
the  folding  of  this  iron  formation.  Therefore  it  is  concluded  that  this 
granite  is  younger  than  the  iron-bearing  formation,  although  the  jasper  is 
in  no  place  cut  by  dikes  which  can  be  connected  directly  with  the  main 
masses  of  the  granite. 

This  g-ranite  is  not  clearly  recognizable  in  the  pebbles  in  the  overlying 
Lower  Huronian  sedimentary  series,  but  where  this  series  comes  closest  to 
the  granite  its  relations  are  sufficiently  clear.  Thus,  for  example,  dikes  of 
this  granite,  as  in  sec.  16,  T.  64  N.,  R.  9  W.,  north  of  Moose  Lake,  are 
found  to  ciTt  the  greenstones  underlying  the  sediments,  but  never  to  pass 
the  contact  and  penetrate  the  sediments,  although  the  dikes  are  numerous 
near  the  contact.  Hence  the  conclusion  is  reached  that  the  granite  of  Trout, 
Burntside,  and  Basswood  lakes  is  older  than  the  Lower  Huronian  sediments. 


262  THE  VERMILION  IKON-BEARING  DISTRICT. 

FOLDING. 

If  the  granite  has  been  subjected  to  severe  mountain-making  processes, 
as  it  presumably  has,  it  does  not  now  show  any  very  marked  effects  of 
these.  No  folding,  of  course,  could  be  traced  in  such  a  homogeneous  rock, 
and  it  is  only  natural  that  one  should  find  an  occasional  shearing  plane  along 
which  the  granite  is  more  or  less  schistose.  In  general  the  granite,  as 
already  stated,  possesses  a  very  massive  character.  Unquestionably  these 
rocks  must  have  taken  part  in  the  folding  of  the  district,  but  the  presump- 
tion is  that  in  general  this  great  granite  mass  bordering  the  north  side  of 
the  Vermilion  district  acted  as  a  relatively  imyielding  area  against  which 
the  rocks  to  the  south  have  been  forced.  As  a  result  partially  of  this,  the 
adjacent  greenstones,  consisting  primarily  to  a  large  extent  of  easily 
alterable  pyroxene,  have  been  metamorphosed  into  amphibolitic  schists. 

INTERESTING   LOCALITIES. 

In  the  following-  paragraphs  will  be  found  brief  descriptions  of  some 
localities  which  show  the  relations  of  the  granite  of  Trout,  Basswood,  and 
Burntside  lakes  to  the  Ely  greenstone. 

The  relations  between  these  rocks  are  very  clearly  shown  at  many 
places  along  the  north  and  west  shores  of  the  northwest  end  of  Pine  Island 
and  on  the  adjacent  shore  of  the  mainland,  where  granite  dikes  intrude  the 
greenstone.  These  dikes  are  scattered  over  a  wide  zone  along  the  contact 
between  these  two  igneous  rocks,  and  as  a  result  of  their  intrusion  the 
greenstone  has  been  altered  to  amphibolitic  schists.  The  dikes  frequently 
contain  large  masses  and  smaller  fragments  of  schists  similar  to  those 
surrounding  them.  About  2  miles  northeast  of  Mud  Creek  Baj-  and  about 
three-fourths  of  a  mile  due  north  of  the  Sheridan  mine,  the  schist  is  intri- 
cately intruded  by  the  granite.  The  schist  has  been  so  broken  up  as  the 
result  of  the  intrusion  that  in  places  there  has  been  formed  almost  an  eruptive 
breccia,  with  the  granite  as  the  cementing  material.  In  some  cases  the 
intrusive  granite  assumes  roundish  forms,  and  where  the  schist  predominates 
one  might  almost  consider  the  rock  a  pseudo-conglomerate  with  granite 
bowlders  in  a  green  schist  matrix. 

The  relationship  between  the  granite  of  Burntside  Lake  and  the 
ellipsoidal  greenstones  is  well  shown  at  a  great  number  of  places  on  the 
shores  of  Bm-ntside  Lake.     Coasting  along  the  southern  shore  to  the  west 


ARCHEAN  GRANITES.  263 

of  the  portage  one  observes  dikes  of  this  granite  in  the  ellipsoidal  green- 
stone, which  has  been  metamorphosed  to  an  amphibolitic  schist  on  the  steep 
north-facing  slopes  in  the  sonthwest  quarter  of  section  23.  Similar  exposures 
occur  again  on  the  point  in  the  southwest  quarter  of  section  22  and  in  the 
southeast  quarter  of  section  21.  The  intrusive  character  of  the  granite  and 
the  intricacy  of  this  intrusion  is  best  shown  on  the  almost  continuous  expo- 
sures that  border  the  northwest  shore  of  the  lake  in  sees.  30  and  20,  T.  63  IST., 
R.  13  W.  One  starting  at  almost  any  place  on  the  southern  shore  of  Burntside 
Lake,  where  the  granite  dikes  are  numerous,  will  find  that  they  diminish  in 
number  southward,  and  with  this  diminution  in  number  at  a  distance  from 
the  main  granite  mass  one  will  find  that  the  schists  gradually  lose  their 
schistosit}'  and  grade  into  the  normal  greenstones. 

Two  hundred  paces  north  of  the  shore  of  Long  Lake,  on  a  line  1,000 
paces  west  of  the  east  line  of  sec.  21,  T.  63  N.,  R.  12  W.,  there  is  a  well- 
marked  eruptive  breccia,  produced  by  the  intrusion  of  granite  of  Burntside 
Lake,  which  includes  a  vast  number  of  fragments  of  the  greenstone  forming 
the  main  country  rock.  A  similar  breccia  is  found  about  70  paces  farther 
north  of  the  above  location.  A  dike  of  the  granite  developed  as  granite- 
porphjay  cuts  the  greenstone  at  1,000  paces  north,  1,000  paces  west  from 
the  southeast  corner  of  sec.  19,  T.  64  N.,  R  10  W. 

The  relationship  between  the  granite  of  Basswood  Lake  and  the  Ely 
greenstone  is  well  shown  on  a  high  hill  at  about  500  paces  north  of  the 
southeast  corner  of  sec.  17,  T.  64  N.,  R.  9  W.,  and  on  the  hill  in  the 
southeast  quarter  of  sec.  16,  T.  64  N.,  R.  9  W.  At  both  of  these  places  the 
greenstone  is  penetrated  by  numerous  dikes,  and  it  has  been  metamor- 
phosed in  most  cases  to  amphibolitic  and  occasionally  micaceous  rock,  in 
which  schistosity  is  very  frequently  more  or  less  well  developed. 

GRANITE  BETWEEN  MOOSE  LAKE  AND  KAWISHIWI  EITER,  IN  SEC.  5, 

T.  6.3  N.,  E.  9  W. 

DISTRIBUTION  AND  EXPOSURES. 

Distribution. — In  the  SE.  {  of  sec.  5,  T.  63  N.,  R.  9  W.,  there  is  an 
oval  mass  of  granite,  ha^^ng  diameters  of  about  one-half  mile  northeast- 
southwest  by  one-fourth  mile  east-west.  In  the  vicinity  of  this  mass  and 
in  the  greenstone  area  for  several  miles  to  the  west — in  general  we  may 
say  in  the  territory  between  Moose  Lake  and  Kawisliiwi  River — there  are 


264  THE  VERMILION  IRON-BEARING  DISTRICT. 

a  number  of  granite  and  granite-porphyry  dikes  which,  since  they  are  of 
practically  the  same  petrographic  character  as  the  large  mass  and  show  the 
same  relationship  to  the  adjacent  rocks  that  this  mass  shows  to  similar  rocks 
adjacent  to  it,  are  presumed  to  be  offshoots  from  this  large  mass,  or  at  least 
to  have  come  from  the  same  deep-seated  mass  of  magma  from  which  it 
came. 

Exposures. — The  exposures  are  fairly  numerous  where  the  large  mass 
of  granite  occurs,  but  over  a  portion  of  the  area  underlain  by  this  there 
is  a  large  amount  of  fallen  timber,  which  helps  to  conceal  the  rocks  and 
renders  the  area  exceedingly  difficult  of  access. 

PETROGRAPHIC  CHARACTERS. 

On  fresh  fracture  this  granite  is  dark  gray  in  color,  though  at  times 
it  has  a  reddish  tinge.  On  weathered  surfaces  it  usually  becomes  grayish. 
It  is  of  medium  grain,  and  is  sometimes  developed  as  a  granite-porphyry 
in  which  the  feldspar  and  quartz  phenocrysts  can  be  easily  seen  lying  in 
the  dark-gra}^  fine-grained  groundmass.  The  granite-porphyry  facies  bears 
a  very  strong  resemblance  to  the  porphyries  of  Vermilion  Lake,  as  well  as 
to  the  porphyritic  facies  of  the  granite  of  Saganaga  Lake.  These  rocks 
show  only  the  ordinary  characters  of  granites,  and  a  brief  description  of 
them  will  suffice.  The  constituents  are  the  iisual  ones — quartz,  orthoclase, 
plagioclase  feldspar  so  altered  that  no  individuals  suitable  for  close  deter- 
minations of  character  could  be  foimd,  a  little  brown  mica,  and  magnetite. 
In  the  porphyry  the  feldspar  is  the  most  prominent  phenocryst,  occurring 
in  both  larger  and  more  numerous  individuals  than  the  rounded  quartz 
phenocrysts  associated  with  it.  The  feldspar  shows  fairly  good  crj^stal  con- 
tours, though  sometimes  the  crystals  are  rounded.  In  the  porphyries  the 
groundmass  in  which  the  phenocrysts  lie  is  a  fine-grained  aggregate  of  feld- 
spar, quartz,  calcite,  epidote,  zoisite,  rutile,  chlorite,  biotite,  sericite,  and 
pyrite,  all  in  small  individuals.  Most  of  these  are  of  secondar}^  origin,  yet 
some  of  the  quartz  and  feldspar,  and  possibly  some  of  the  biotite  may  be 
primary,  although  not  recognizable  as  such. 

RELATIONS  TO  ADJACENT  FORMATIONS. 

In  immediate  proximity  to  the  main  mass  of  this  granite  are  the  Ely 

greenstone  of  the  Archean  and  sedimentaries  of  Lower  Huronian  age  only. 

Relation  to  Archean. — In  the  vicinity  of  the  granite  the  Archean  green- 


ARCHEAN  GRANITES.  265 

stone  is  cut  by  acid  dikes  which  are  petrograpliically  similar  to  this  granite 
and  are  beheved  to  be  offshoots  from  it.  Hence  the  fact  that  the  Ely  green- 
stone is  older  than  the  granite  is  indisputably  shown. 

Although  the  Soudan  formation  does  not  occur  near  the  main  mass  of 
the  granite,  nevertheless  dikes  similar  to  the  granite  are  found  cutting 
through  this  iron  formation  at  places  a  number  of  miles  distant  from  the 
main  massive,  and  if  it  is  admitted  that  these  dikes  belong  to  the  same 
period  of  intrusion  as  the  main  mass  of  granite,  then  it  is  equally  plain  that 
the  iron  formation  is  older  than  the  granite. 

Relation  to  Lower  Huronian. — The  Lower  Hui'onian  sediments  and  the 
granite  occur  close  to  each  other.  For  the  most  part  these  sediments  are 
fine  slates  with  graywackes  and  very  few  narrow  bands  of  cong-lomerate. 
However,  at  a  few  places  conglomerates  have  been  found  overlying-  and 
derived  from  the  granite,  and  very  good  proof  of  the  relations  between 
the  granite  and  Lower  Huronian  sediments  is  thereby  given.  Negative 
evidence  is  furthermore  offered  by  the  fact  that  no  dikes  which  can  be 
identified  with  the  granite  are  found  penetrating  these  Lower  Huroniai^ 
sediments,  although  they  occur  in  the  greenstones  that  immediately  underlie 
these  sediments,  and  are  in  close  proximity  to  them. 

Relation  to  Keiveenawan. — The  granite  is  itself  cut  by  narrow  dikes  of 
coarse  black  diabase,  which  are  supposed  to  be  of  Keweenawan  age,  the 
very  youngest  intrusives  occurring  in  the  district. 

GRAlSriTE  OF  SAGAKAGA  LAKE. 

The  granite  of  Saganaga  Lake  has  probably  one  of  the  best-known 
names  of  any  geologic  formation  occurring  in  the  Vermilion  district  of 
Minnesota,  for  it  has  appeared  repeatedly  in  the  Minnesota  reports  and  in 
other  publications  in  which  its  field  and  age  relations  to  the  adjacent  rocks 
have  been  discussed." 

"Winchell,  A.,  Geol.  and  Nat.  Hist.  Survej'  of  Minnesota,  Sixteenth  Ann.  Rept.,  1888,  pp. 
211-233  and  330-334.  Grant,  U.  S.,  Geol.  alid  Nat.  Hist.  Survey  of  Minnesota,  Twentieth  Ann.  Kept., 
1893,  pp.  83-95;  Final  Rept.,  Vol.  IV,  1899,  pp.  321-323  and  467;  also  Am.  Geologist,  Vol.  X,  1892, 
p.  7.  Lawson,  A.  C,  Am.  Geologist,  Vol.  VII,  1891,  p.  324.  Geological  age  of  the  Saganaga  granite, 
by  H.  V.  Winchell:  Am.  Jour.  Sci.,  3d  series.  Vol.  XLI,  1891,  pp.  386-390. 


266  THE  VERMILION  IRON-BEARING  DISTRICT. 

DISTRIBUTION,  EXPOSURES,  AND  TOPOGRAPHY. 

Distribution. — This  gi-anite  is  confined  in  its  oeciuTence  to  Saganaga 
Lake  and  its  vicinity.  It  covers  about  100  square  miles  in  Minnesota,  but 
only  a  portion  of  its  area  is  shown  on  the  accompanying  map  (PI.  II). 

Exposures. — Within  this  area  its  exposures  are  very  numerous,  for  the 
country  is  in  mau}^  places  comparatively  bare  of  vegetation,  the  drift  is  as 
a  rule  thin,  and  the  presence  of  large  bodies  of  water — Saganaga,  AVest 
Gull,  and  Red  Rock  lakes  and  their  tributary  streams — insure  frequent 
exposm'es  on  the  shore  of  the  mainland  and  on  the  islands. 

Topography. — Tlie  topography  of  tliis  granite  area  offers  a  strong  con- 
trast to  that  of  the  surrounding  country.  In  the  smTOunding-  territory, 
which  is  underlain  by  Archean  greenstones  and  Lower  Huronian  sediments, 
the  topography  is  roug'h,  being-  marked  b)^  prominent  hills,  generally,  to  be 
sure,  having  the  round  contours  characteristic  of  glaciated  areas,  but  often 
presenting  high  and  rugged  cliffs.  Within  the  granite  area  the  topographic 
features  are,  for  the  most  part,  not  strongly  emphasized.  The  hills  are  low 
and  rounded  and  the  hilltops  seem  to  approach  very  nearly  the  same  level, 
so  that  in  looking  over  this  area  from  some  of  the  higher  surrounding- 
elevations  one  g'ets  an  idea  that  this  particular  portion  of  the  district  has 
been  reduced  very  nearly  to  a  peneplain. 

PETROGRAPHIC  CHARACTERS. 

Macroscopic  characters. — This  granite  is  very  coarse  g-rained  over  a  large 
portion  of  the  area  in  which  it  occurs  and  is  usually  developed  as  a  grajiite- 
porphvry  in  which  the  phenocrysts  are  large  quartzes.  However,  as  one 
traverses  the  region  from  West  Grull  Lake  through  Red  Rock  to  Saganaga 
Lake — that  is,  as  one  goes  approximately  from  the  periphery  toward  the 
center  of  the  area — it  is  very  noticeable  that  the  grain  of  the  rock,  which  is 
relatively  fine  upon  the  exposvires  on  West  Gvill  Lake,  gi'ows  coai'ser  toward 
the  center.  This  is  one  of  the  evidences  in  favor  of  the  intrusive  character 
of  the  granite.  In  color  the  granite  varies  from  light  gray  to  pink,  and 
even  to  brick  red.  This  last  strong  tint  is  usually  present  where  the 
alteration  is  the  most  pronounced. 

The  granite  massive  is  cut  in  various  places  by  fiue-gTained  red  aplite 
dikes. 


ARCHEAN  GRANITES.  267 

Here  and  there  in  the  granite  may  be  found  masses  of  dark  gray  to 
green  rock.  Some  of  them  have  been  in  planes  along  which  movement  has 
taken  place,  and  are  extremely  altered,  and  have  become  schistose.  Others 
have  been  much  altered,  but  no  actual  motion  appears  to  have  occurred  in 
them,  so  that  they  are  still  massive.  These  rocks  are  generally  basic- — 
some  are  even  ultrabasic — and  vary  from  basalts  to  peridotites.  They 
are  intrusive  in  the  granite,  but  how  much  younger  than  the  granite  they 
may  be  is  not  known.  The  schistose  basic  rocks  presumably  belong'  to 
a  period  of  eruption  later  than  the  Lower  Huronian,  and  correspond  to 
the  dikes  described  in  Chapter  IV.  The  freshest  basalts  are  presumably 
of  Keweenawan  age  and  are  similar  to  those  described  in  Chapter  VI, 
under  the  heading  "Keweenawan." 

Microscopic  characters. — Under  the  microscope  the  granite  is  found  to 
be  either  a  mica-  (biotite-)  granite  or  a  hornblende-granite.  This  last  is  the 
predominant  rock.  It  varies  by  loss  of  quartz  to  a  syenite.  Glrant"  has 
described  still  a  different  facies  of  the  granite  of  Saganaga  Lake — a  fluorite- 
granite  which  he  observed  upon  an  island  in  Saganaga  Lake.  The  essential 
minerals  are  mica,  hornblende,  quartz,  orthoclase,  and  plagioclase.  With 
these  occur  as  accessory  minerals,  and  in  very  small  quantity,  some  apatite, 
sphene,  and  magnetite.  These  possess  their  usual  characters  and  show  the 
relations  common  to  such  minerals  in  the  granites.  All  of  the  rocks  are 
considerably  altered.  The  usual  secondary  minerals — calcite,  sericite, 
actinolite,  epidote,  and  chlorite — have  been  produced  and  are  present  in  the 
sections  examined.  The  granite  varies  somewhat  in  textural  character  from 
the  normal  granite  to  a  granite-porphyry. 

In  the  granite-porphyries  the  phenocrysts  are  quartz,  hornblende,  and 
plagioclase.  Around  some  of  the  feldspar  phenocrysts  there  is  occasionally 
micropegmatitic  intergrowth  of  the  feldspar  with  quartz.  These  minerals 
are  distinctly  of  the  first  generation,  and  lie  in  a  moderately  fine-grained 
groundmass  of  hornblende,  feldspar,  and  quartz  of  the  second  generation, 
in  striking  contrast  in  size  to  those  of  the  first  generation. 

"Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Twentieth  Ann.  Rept,  1893,  p.  89;  Geol.  and  Nat. 
Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV,  1899,  p.  323. 


268  THE  VERMILION  IRON-BEARING  DISTRICT. 

RELATIONS   TO   ADJACENT  FORMATIONS. 

The  granite  of  Saganaga  Lake  is  found  in  contact  with  and  showing 
clearly  its  relations  to  the  Archean  Ely  greenstone  and  the  Lower  Huronian 
sediments — the  Ogishke  conglomerate,  and  the  Knife  Lake  slates. 

Relations  to  Ely  greenstone. — In  the  southern  portion  of  the  area  under- 
lain by  the  granite  of  Saganaga  Lake,  on  the  south  shores  of  West  Gull  and 
Gull  lakes,  and  along  the  contact  between  the  granite  and  the  Ely  green- 
stone to  the  east  of  these  localities,  the  granite  penetrates  the  Archean 
greenstones  in  numerous  dikes.  Moreover,  the  intrusive  nature  of  the 
granite  is  further  shown  by  the  fact  that  in  the  contact  zone  the  greenstone 
is  metamorphosed  by  the  granite  to  an  amphibolitic  schist,  whereas  at  some 
distance  away  from  the  contact  zone — that  is,  beyond  the  influence  of  the 
granite — the  greenstones  show  their  normal  characters. 

On  the  northern. side  of  the  granite  area,  on  the  north  and  east  shores 
of  Cache  Bay  of  Saganaga  Lake  (this  is  within  Canadian  territory),  the 
same  relations  are  clearly  shown  on  a  great  number  of  exposures  around 
the  shores  of  the  bay.  Here,  too,  the  metamorphism  of  the  greenstones 
diminishes  as  the  distance  from  the  main  mass  of  granite  increases.  Further- 
more, the  granite  contains  inclusions  of  rock  derived  from  this  greenstone. 

On  Red  Rock,  West  Gull,  and  Gull  lakes  there  are  in  places  in  the 
granite  irregular  fragments  of  hornblendic  rocks  that  are  believed  to  have 
been  derived  from  the  ancient  greenstones  tlii'ough  which  the  granites  were 
intruded.  This  intrusive  relationship  of  the  granite  and  greenstone  has 
been  recognized  by  all  geologists  who  have  studied  this  area,  except  H.  V. 
Winchell,"  who  maintains  that  the  granite  is  derived  from  the  greenstones, 
or  Keewatin  green  schists,  as  he  calls  them. 

Relations  to  the  Lower  Huronian  sediments. — No  such  general  agreement 
has  been  reached  among  the  geologists  who  have  studied  the  Vermilion 
district  and  the  adjacent  district  in  Ontario  as  to  the  relationship  which 
exists  between  the  granite  of  Saganaga  Lake  and  the  adjacent  sedimentaries. 
A.  H.  WinchelP  has  decided  that  the  granite  is  younger,  as  a  granite,  than 
the  sedimentaries,  and  that  it  was  derived  from  them  by  processes  of 
progressive  metamoi-phism.     Lawson"  describes  it  as  intrusive  in  the  sedi- 

«  Geological  age  of  the  Saganaga  syenite,  by  H.  V.  Winchell:  Am.  Jour.  Sci.,  3d  Series,  Vol.  XLI, 
1891,  p.  389. 

f>  Winchell,  A.,  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Sixteenth  Ann.  Kept,  1888,  p.  211. 
'Lake  Superior  stratigraphy,  by  A.  C.  Lawson:  Am.  Geologist,  Vol.  VII,  1891,  p.  324. 


ARCHEAN  GRANITES.  269 

mentaries,  and  having  no  other  connection  with  them.  Gi'ant,"  having  first 
considered  the  granite  as  intrusive  in  the  sedimentaries,  examined  it  a 
second  time,  and  since  then  has  maintained  that  the  sedimentaries  are 
younger  than  the  granite,  having  been  derived  from  it. 

The  sedimentaries  lie  on  the  western  flank  of  the  granite  area  extend- 
ing from  Cache  Bay  of  Saganaga  Lake  on  the  north  to  West  GruU  Lake  on 
the  south.  Over  a  considerable  portion  of  this  area — for  instance,  where 
the  drainage  is  imperfect — exposures  are  very  few,  thick  morainal  deposits 
covering  that  part  of  the  area  extending  approximately  from  sec.  30,  T.  6G 
N.,  R.  5  W.,  southward  into  sec.  5,  T.  65  N.,  R.  5  W.  At  the  extreme  north 
and  the  extreme  south,  however,  Saganaga  and  West  Gull  lakes,  respec- 
tively, lie  along  the  contact  and  give  fairly  good  opportunities  for  a 
study  of  the  existing  relations.  On  the  northern  exposures  especially  the 
relations  are  so  absolutely  clear  and  convincing  that  the  phenomena  there 
observed  will  be  first  described. 

Following  the  international  boundary  route  from  west  to  east,  one 
passes  in  order  through  the  long,  narrow  lakes  of  Knife  and  Otter  Track 
(Cypress),  then  through  Oak  (Swamp)  Lake  into  a  bay  of  Saganaga  Lake. 
The  rocks  exposed  on  these  lakes  are  chiefly  slates  and  graywackes,  with 
occasionally  a  fine  interstratified  conglomerate.  On  Knife  Lake  the  strike 
of  the  slates  is  about  N.  70^  to  80°  E.  As  we  go  eastward  we  note  a 
change  in  this  strike,  and  when  the  east  end  of  Otter  Track  Lake  is  reached 
the  strike  has  become  N.  45°  E.  to  N.  20°  E.  and  N.  10°  E.,  and  even  in 
places  is  shown  as  nearly  north  and  sojath.  From  Otter  Track  Lake  we 
cross  on  the  portage  a  ridge  of  the  slates  and  then  enter  Oak  Lake  (Swamp 
Lake),  where  there  are  exposed  over  the  greater  portion  of  the  shores  the 
same  dark  slates  and  g-raywackes  that  are  fotnid  on  the  lakes  farther  west. 
On  this  lake  the  strike  of  these  sediments  has  tui'ned  until  it  is  west  of 
north.  On  the  east  side  of  the  lake  the  sediments  are  noticeably  difl^erent 
in  character  from  those  we  have  been  observing.  They  are  no  longer 
dark,  but  are  light  in  color — pink  to  reddish — and  instead  of  being  fine 
slates  are  predominantly  coarse-grained  arkoses.  They  show  distinct 
bedding  and  dip,  and  one  can  trace  gradations  from  the  coarsest-grained 
rocks  into  the  finer-grained  ones.  This  alternation  was  noted  bv  earlier 
observers,  but  was  misinterpreted.     These  coarse  ai'koses  so  closely  resemble 

"Grant,  U.  S.,  Geol.  and  Nat.  Hist.  Survey  of  JMinnesota,  Final  Kept.,  Vol.  IV,  1899,  p.  322. 


270  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  granite  of  Sagauaga  Lake  (they  are  derived  directly  from  it)  that  they 
were  actuall}'  taken  for  that  granite,  altered,  however,  and  were  supposed 
to  have  been  intruded  iu  thin  sheets  parallel  to  the  fine-grained  beds 
lying  in  alternation  with  them.  This  was  Grant's  idea  upon  his  fii-st 
visit,  when  he  decided  that  the  granite  of  Saganaga  Lake  was  for  this 
reason  j'ounger  than  the  adjacent  sediments."  A  subsequent  visit  caused 
him  to  change  his  view  to  the  correct  one,  upon  recognition  of  their  true 
relations.  The  sedimentary  characters  of  these  bedded  arkoses  were  noted 
by  N.  H.  Winchell,*  and  likewise  the  resemblance  of  these  arkoses  to 
the  granite  of  Saganaga  Lake,  the  main  mass  of  which  lies  east  of  and 
beneath  these  sediments,  and  the  relations  were  interpreted  by  him  as 
evidence  of  progressive  downward  metamorphism,  the  arkoses  having 
been  fused  and  transformed  into  the  granite  of  Saganaga  Lake.  Examined 
under  the  microscope,  these  bedded  rocks  are  seen  to  be  made  up  of 
fragments  of  quartz  and  feldspar  and  flakes  of  mica.  None  of  the  minerals 
are  well  rounded,  but  neither  do  they  show  the  same  relationships  to  each 
other  that  the  same  minerals  always  exhibit  in  unquestionable  granites 
Moreover,  the  well-marked  sedimentary  banding  iu  them  and  the  gradation 
from  coarser-  to  finer-grained  rocks  show  that  these  are  without  question 
fragmental  rocks  or  arkoses  derived  from  the  granite  and  consisting  of  the 
same  constituents  as  the  granite  from  which  they  were  derived.  These 
fragments  of  minerals  have  riot  been  very  much  worn,  and,  since  they 
were  deposited  here  through  the  action  of  water,  have  been  cemented 
together  so  as  to  form  a  rock  that  is  strikingly  like  a  granite,  especially  to 
one  making  a  superficial  macroscopic  examination.  A  microscopic  study, 
and  indeed  a  close  macroscopic  field  study,  immediately  discloses  the 
characters  above  mentioned,  and  shows  that  they  are  different  from  the 
granite.  Excellent  exposures  of  the  arkose  occur  on  the  portage  from  Oak 
Lake  to  the  west  bay  of  Saganaga  Lake,  and  good  exposures  of  the  frag- 
mentals  showing  distinct  bedding  may  be  seen  on  tlie  north  shore  of  this 
bay,  on  the  point  just  east  of  the  portage.  After  passing  along  these 
outcrops  of  sediments  one  finally  comes  to  the  clearly  recognizable  typical 
massive  granite-porphyry  of  Saganaga  Lake.  These  rocks  have  the 
distribution  shown  upon  Sheet  XVI  in  the  accompanying  atlas. 

"Geo),  and  Nat.  Hist.  Survey  of  Minnesota,  Twentieth  Ann.  Rept.,  189.S,  pp.  90-95.         ''Loo.  cit. 


ARCHEAN  GRANITES.  271 

Farther  east  and  north  on  the  shores  of  Cache  Bay  of  Saganaga  Lake 
the  relations  between  the  granite  and  these  sediments  are  shown  still  more 
conclusively,  if  that  be  possible,  than  at  the  localities  above  referred  to. 
On  the  southwest  side  of  Cache  Bay,  at  a  number  of  excellent  exposures, 
the  granite  of  Saganaga  Lake  is  overlain  to  the  west  by  a  beautiful  typical 
coarse  basal  conglomerate.  This  conglomerate  is  made  up  almost  solely 
of  bowlders  of  all  sizes,  derived  directly  from  the  immediately  adjacent 
granite  of  Saganaga  Lake.  The  matrix  between  these  larger  fragments  is 
the  finer  detrital  material  derived  from  the  same  source. 

The  unconformable  relations  of  the  sediments  known  as  the  Ogishke 
conglomerate  and  the  Knife  Lake  slates,  here  classed  as  Lower  Huronian, 
to  the  granite  of  Saganaga  Lake  and  the  Ely  greenstones  of  this  eastern  por- 
tion of  the  district  are  clearly  shown  by  the  above-stated  field  occurrences. 

On  West  Gull  Lake  exposures  are  not  nearly  so  good  nor  so  extensive 
as  on  Saganaga  Lake.  Nevertheless,  from  a  study  of  this  area  correct  con- 
clusions concerning  the  relations  of  the  rocks  were  reached,  and  the  later 
study  of  the  Saganaga  Lake  area  merely  served  to  emphasize  their  accu- 
racy. The  areal  distribution  of  the  rocks  within  a  small  part  of  the 
district  on  the  west  shore  of  West  Gull  Lake  is  shown  on  the  accompany- 
ing map,  fig.  17.  Starting  in  at  the  granite  outcrop  between  the  two 
meander  corners  at  a  point  about  one-fourth  of  a  mile  north  of  the  southeast 
corner  of  sec.  7,  T.  6.5  N.,  R.  5  W.,  we  find  that  the  granite  along  the  shore 
shows  its  normal  characters  and  is  cut  by  several  dikes  of  basic  rock.  As 
we  follow  the  exposure  inland,  however,  the  granite  is  fotmd  to  become 
more  and  more  schistose,  and  finally  we  notice  that  this  schistose  rotten  rock 
is  made  up  of  small  gi-anitic  fragments,  with  the  finer  granitic  debris  for 
cement.  Its  fragmental  character  is  most  clearly  shown  by  an  occasional 
very  small  fragment  of  jasper.  In  examining  this  exposure  one  can  say, 
when  the  extremes  are  seen,  "Here  is  a  granite,  and  here  is  a  clastic  derived 
from  the  granite ; "  but  no  sharp  line  of  demarcation  between  them  can  be 
drawn,  for,  indeed,  there  is  no  such  sharp  line.  On  the  contrary,  there  is 
an  imperceptible  gradation  from  the  one  to  the  other  through  the  interme- 
diate schistose  material  which  probably  represents  the  disintegrated  portion 
of  the  granite  which  was  not  removed  by  erosion.  In  several  other  places, 
where  the  granite  and  sedimentaries  come  nearl}^  together,  they  are  se-pa- 
rated  by  a  narrow  area  usually  marked  by  a  small  topographic  depression 


272 


THE  VERMILION  IRON-BEARING  DISTRICT. 


in  which  uo  rocks  are  exposed  or  in  which  there  is  only  an  occasional  iso- 
lated exposure  of  rotten  schistose  rock.     In  such  exposures  the  characters 


Corner 


ALGONKIAN 


UOWER      I^URONIAN 

Ogishke  conglomerate 


ARCHEAN 


Granite  Granite  porphyry 


(with  observed  (without observed 

strike  and  dip)  strikeand  dip) 

Scale 
O  1/4. 


'/a  mile 


Fio.  17.— DL'tiiil  geologic  map  sliuwing  exposures  in  u  smiill  iiruii  on  West  linll  Luke. 


ARCHEAN  GRANITES.  273 

of  this  rock  can  not  be  definitely  recognized  in  all  cases,  but  since  it  is 
analogous  in  all  respects  to  the  material  described  above  as  occurring 
between  the  granite  and  the  clearly  recognizable  sediments,  it  is  assumed 
to  be  the  schistose  arkose  that  lies  between  the  granite  and  the  overlying 
sediments.  These  sediments  consist  of  interbedded  slates,  gray  wackes,  and 
conglomerates,  and.  in  these  conglomerates  pebbles  were  observed  which 
could  be  identified  with  the  granite  of  Saganaga  Lake.  After  a  study  of 
the  exposures  here  there  can  be  no  reasonable  doubt  that  the  sediments  are 
younger  than,  and  partially  derived  from,  the  adjacent  granite  of  Saganaga 
Lake. 

Within  the  area  shown  on  the  map  forming  fig.  17  there  is  a  knob  of 
greenstone,  penetrated  by  numerous  dikes  of  g-ranite,  similar  to  those 
occurring  in  the  greenstone  adjacent  to  the  granite  of  Saganaga  Lake  at 
other  places  in  this  district.  These  dikes  are  presumed  to  be  offshoots  from 
the  granite.  Overlying  this  granite  are  sediments  similar  to  those  overljdng 
the  not  far  distant  granites,  consisting  to  a  considerable  extent  of  pebbles 
of  granite  and  greenstone,  showing  them  to  be  younger  than  both  the 
greenstones  and  the  granite.  Within  this  small  area,  therefore,  we  find 
the  Archean  greenstone,  the  granite  of  Saganaga  Lake,  and  the  Lower 
Huronian  sediments,  with  their  relations  to  one  another  clearly  shown. 
The  Lower  Huronian  sediments  are  now  folded  into  synclines  within  the 
granite  and  greenstone,  and  hence  wrap  around  these  rocks,  as  is  shown 
on  the  accompanying  map  (fig.  17). 

METAMORPHIC  EFFECTS  OF  THE  GRANITE  OF  SAGANAGA  LAKE. 

The  granite  of  Saganaga  Lake  having  been  found  intrusive  only  in  the 
greenstones  of  Archean  age,  we  are  able  to  study  its  metamorphic  effects 
upon  these  rocks  alone.  These  effects  are  in  all  respects  the  same  as  those 
produced  upon  the  similar  greenstones  by  the  intrusion  of  the  granites  of 
Trout,  Basswood,  and  Burntside  lakes,  as  the  result  of  which  amphibolitic 
schists  were  produced.  The  processes  of  metamorphism  induced  by  these 
intrusions  and  the  products  resulting  therefrom  have  been  described  in 
preceding  portions  of  this  monograph  (p.  156  et  seq.). 

INTERESTING  LOCALITIES. 

Exposures  of  the  granite  of  Saganaga  Lake  are  so  extensive  in  the 
area  in  which  it  occurs  that  it  is  unnecessary  to  refer  to  any  special  locality 
at  which  its  characters  may  be  studied.     There  are,  however,  several  places 
MON  XLV — 03 18 


274  THE  VERMILION  IKON-BEARING  DISTRICT. 

at  which  the  relations  between  the  granite  and  the  adjacent  rocks  may  be 
noted  with  advantage,  and  although  these  have  already  been  mentioned, 
attention  will  be  again  called  to  them. 

The  relations  of  the  granite  of  Saganaga  Lake  to  the  Ely  ellipsoidal 
greenstone  may  be  seen  at  almost  any  place  along  the  contact  between  the 
two  on  the  south  shore  of  Gull  Lake.  For  instance,  just  below  the  north 
section  lines  of  sees.  22  and  23,  T.  65  N.,  R.  5  W.,  numerous  dikes  of  the 
granite  cut  the  greenstone.  The  greenstone  near  the  contact  with  the 
granite,  where  it  is  full  of  granite  dikes,  has  been  extremely  metamorphosed. 
The  farther  we  go  southward  from  the  contact  the  less  altered  is  the  green- 
stone and  the  better  preserved  are  the  ellipsoidal,  amygdaloidal,  and  other 
structm-es.  The  southwest  shore  of  West  Gull  Lake  and  the  small  lake  on 
the  portage  route  between  West  Gull  and  Gull  lakes  are  easily  accessible, 
and  here  in  the  cliffs  many  dikes  of  granite  cut  the  greenstone.  The  same 
relation  is  very  clearly  shown  on  the  northeast  shore  of  Cache  Bay,  which 
is  the  large  bay  of  Saganaga  Lake  that  extends  into  Canada.  Along  this 
shore  innumerable  dikes  of  the  granite  cut  these  schists. 

The  granite  of  Saganaga  Lake  is  found  in  contact  also  with  the  Ogishke 
conglomerate,  and  its  relation  to  the  Ogishke  conglomerate  is  well  shown  at 
certain  places  (mentioned  above,  pp.  269-273)  on  the  west  side  of  West  Gull 
Lake,  on  Saganaga  Lake,  and  on  Cache  Bay  of  Saganaga  Lake.  At  all  of 
these  places  the  conglomerate  consists  largely  of  bowlders  and  finer  detrital 
material  derived  from  the  granite.  The  rocks  along  the  contact  have  in 
places  been  closely  folded,  and  as  a  result  of  this  folding  the  contact 
between  the  two  is  somewhat  irregular  and  the  relations  appeared  to  be 
complicated,  but  careful  studies  of  the  exposures  have  shown  the  relations 
above  stated. 


i 


CHAPTER  IV. 

THE   LOWER   HURONIAN. 

SECTION  L— SEDIMENTARY  ROCKS. 

OCCURRENCE   AIN^D    SUBDIVISIOIS^S. 

The  Lower  Huroniau  sediments  of  the  Vermilion  district  have  a  very 
large  surface  extent.  They  are  present  in  two  large  detached  areas — one, 
known  as  the  Vermilion  Lake  area,  extending  eastward  from  the  western 
limit  of  the  area  mapped  near  Tower,  on  Vermilion  Lake,  to  within  about  1 1 
miles  of  Ely;  the  other,  known  as  the  Knife  Lake  area,  beginning  about 
7  miles  west  of  Ely  and  extending  eastward  to  the  eastern  limit  of  the  area 
mapped.  These  same  rocks  extend  farther  eastward  for  an  unknown  but 
great  distance,  passing  north  of  and  around  the  granite  of  Saganaga  Lake 
into  Canada.  Where  the  Vermilion  and  Knife  Lake  areas  approach 
each  other — that  is,  west  of  Ely — the  rocks  have  their  least  surficial  extent, 
rapidly  widening  as  we  follow  them  from  this  point  eastward  or  westward. 
This  distribution  is  due  to  the  fact  that  the  sediments  occur  in  two  great 
synclinoria.  The  short  distance  of  about  5  miles  by  which  the  continuity 
of  the  rocks  is  interrupted  represents  the  place  where,  as  a  result  of  a 
cross  anticline,  the  lower  (Archean)  rocks  have  been  brought  to  the  sui-face. 
This  gap  is  so  narrow,  and  the  structure  points  so  clearly  to  the  original 
extension  of  the  sediments  across  it,  that  this  lack  of  continuity  is  not  con- 
sidered important.  Clearly  the  rocks  of  the  two  areas  were  continuous 
before  erosion  separated  them.  Considered  in  a  broad  way,  the  sediments 
of  the  Lower  Huronian  are  fragmental  rocks  consisting  predominantly  of 
conglomerates  and  slates,  although  fragmentals  intermediate  between  con- 
glomerates and  slates  are,  of  course,  present. 

It  is  difficult  to  estimate  the  relative  quantity  of  the  several  kinds  of 
elastics  included  in  the  sediments.  Moreover,  they  differ  in  respective 
quantity  in  different  parts  of  the  area.  The  conglomerates  form  by  far  the 
more  striking  portion,  and  the  casual  visitor  to  the  district  will  notice 
beautiful  exposiires  studded  with  brilliant-red  jasper  pebbles,  and  draw, 


276  THE  VERMILION  IRON-BEARING  DISTRICT. 

})erhaps,  the  conclusion  that  the  conglomerate  is  the  predominant  clastic, 
but  it  is  believed  the  slates  make  u]}  the  greater  part  of  the  sediments. 

Included  within  the  Lower  Huronian  there  is  a  horizon  of  iron-bearing 
carbonates.  These  carrj^  a  considerable  quantity  of  iron  as  carbonate  in 
addition  to  the  calcium-magnesium  carbonates.  There  is  also  developed  at 
a  few  localities  an  iron-bearing  formation  consisting  of  banded  jasper, 
cherts,  and  iron  ore.  This  iron  formation  is  present  in  very  small 
quantity,  and  certainly  will  never  be  of  any  importance  on  the  United 
States  side  of  the  international  boundary.  These  two  kinds  of  rock  are 
presumed  to  correspond  to  each  other — that  is,  they  belong  to  the  same 
horizon.  On  the  scale  on  which  the  map  is  published,  it  would  be  impossible 
to  represent  all  of  the  different  bauds  of  conglomerates,  grits,  slates,  etc. 
Consequently  no  attempt  has  been  made  to  discriminate  between  these  kinds 
of  the  fragmental  rocks  further  than  to  show  the  ai'eal  distribution  of  the 
extremes. 

We  are  enabled  to  divide  the  Lower  Huronian  sediments  into  thi-ee 
parts — (1)  a  lower  division,  which  is  predominantly  conglomeratic  and 
which  is  most  typically  developed  near  Ogislike  Muncie  Lake,  and  is  called 
the  Ogishke  conglomerate;  (2)  a  division  represented  only  in  the  eastei-n 
portion  of  the  district,  consisting  of  iron-bearing  rocks  and  known  as  the 
Agawa  formation;  and  (3)  a  division  which  is  predominantly  a  slate 
formation  and  which  we  shall  denominate  the  Knife  Lake  slates,  since 
these  slates  are  well  developed  and  splendidly  exposed  on  and  near 
Knife  Lake.  Mention  has  already  been  made  of  the  fact  that  the  Lower 
Huronian  sediments  occur  in  two  separate  areas  within  this  district.  In 
each  area  both  the  conglomerate  and  slate  are  well  developed.  There  are, 
however,  certain  local  differences  in  the  rocks  that  underlie  the  Lower 
Huronian,  and  as  a  consequence  the  sediments  in  the  two  areas  are 
slightly  different.  For  this  reason,  and  also  as  it  simplifies  exposition,  it 
is  considered  best  to  describe  the  rocks  of  the  two  areas  separately.  It 
must  in  each  case  be  clearly  understood  that  the  conglomerates  of  the  two 
areas  are  geologically  contemporaneous  and  that  the  same  contem^joraneity 
exists  in  the  case  of  the  slate  formation.  The  Agawa  formation  is  present, 
however,  only  in  the  Knife  Lake  area  and  can  not  be  correlated  with  any 
definite  formation  in  the  western  area.  The  areas  will  be  described 
separately  in  the  following  pages. 


THE  LOWER  HURONIAN.  277 

VEBMILIOIf   LAKE   AREA  OF   THE  LOWER   HUROISTIAN   SEDIMENTS. 

DISTRIBUTION,  EXPOSURES,  AND  TOPOGRAPHY. 

Listribution. — The  Lower  Huroniaii  of  this  area  has  been  found  to 
extend  very  much  farther  west  than  it  is  shown  to  do  on  the  accompanying 
map  (PI.  II).  It  has  been  carefully  studied,  however,  only  in  the  area  out- 
lined thereon,  and  in  this  description  we  must  consider  it  as  beginning'  at  the 
western  limit  of  the  map,  where  the  rocks  of  the  series  cover  a  very  broad 
area,  corresponding  in  width  23i'actically  with  the  width  of  the  map.  Its 
greatest  breadth  is  something  like  11  miles  in  Ts.  61,  62,  and  63  N.,  R.  16 
W.  As  this  area,  as  outlined  upon  the  map,  is  followed  to  the  east,  we  note 
that  it  is  subdivided  into  a  number  of  smaller  areas  by  the  various  fingers  of 
Archean  rocks  that  project  westward  into  it.  Beginning  at  the  south,  the 
area  underlain  by  Lower  Huronian  rocks  is  found  to  extend  eastwai'd  very 
nearly  to  Bear  Head  Lake,  and  on  the  north  from  the  Archean  near  West  Two 
Rivers  southward  to  the  limit  of  the  area  mapped.  Indeed,  a  reconnaissance 
shows  conclusively  that  the  same  sediments  continue  beyond  the  limits  of  the 
map,  and  are  practically  bordered  on  the  south  by  the  Griants  Range  granite. 
The  next  area  north  of  this  projects  eastward  only  so  far  as  the  town  of 
Tower.  It  is  but  a  short  tongue,  and  is  bounded  on  the  south  by  the 
Archean  greenstone  and  on  the  north  by  this  greenstone  and  the  associated 
iron-bearing  formation  which  constitutes  Lee  and  Tower  hills.  North  of 
this  there  is  a  third  tongue,  occupying  the  valley  between  the  anticlines  of 
Tower  and  Soudan  hills,  projecting  eastward  at  least  as  far  as  the  village 
of  Soudan.  North  of  Soudan  Hill,  and  occupying  in  general  the  basin 
in  which  Vermilion  Lake  lies,  there  occurs  the  main  portion  of  the  area 
underlain  by  the  Lower  Huronian  sediments.  This  is  also  that  part  of  the 
area  in  which  the  best  exposures  occur.  The  rocks  of  this  area  have  been 
followed  eastward  as  far  as  sees.  2  and  16,  T.  62  N.,  R.  14  W.  The  main 
area  of  the  Lower  Huronian  sediments  around  Vermilion  Lake  may  be 
subdivided  into  a  number  of  smaller  areas,  due  to  the  structural  relations 
of  the  rocks.  On  the  east  shore  of  Vermilion  Lake,  for  example,  there  are 
a  number  of  smaller  tongues  into  which  the  area  can  be  divided.  These 
will  not  here  be  described  in  detail,  but  may  be  found  outlined  on  the 
maps  in  the  accompanying  atlas.  The  length  of  this  Lower  Huronian 
belt  from  the  western  limit  of  the  area  mapped  to  the  eastern  end  of  the  belt 
in  which  the  exposures  occur  is  about  17  miles.      The  Lower  Hui'onian 


278  THE  VERMILION  IRON-BE AKING  DISTRICT. 

sediments  within  this  area  are  subdivisible  into  conglomerates  and  slates,  the 
formations  occiuTing  intermediate  between  these  having  been  classed  with 
one  or  the  other,  according  to  the  predominance  of  the  one  or  other  kind  of 
rock  in  the  outcrops.  The  conglomerate  of  this  area  has  been  called  the 
Stuntz  conglomerate."  However,  the  two  subdivisions  of  the  series  in  the 
Vermilion  Lake  area  are  correlative  with  the  Ogishke  conglomerate  and 
Knife  Lake  slates  of  the  typical  areas  in  the  eastern  part  of  the  Vermilion 
district,  and  will  be  called  by  the  same  name  in  the  description  of  the 
western  area. 

Exposures. — On  the  islands  and  on  the  shores  of  Vermilion  Lake  the 
exposures  of  the  conglomerate  are,  on  the  whole,  excellent,  and  are  both 
frequent  and  of  large  size.  In  the  inland  areas,  however,  the  exposures 
are  not  so  numerous  and  are  usually  small. 

The  slates  are  not  so  well  exposed  on  the  islands  and  shore  of 
Vei'milion  Lake  as  are  the  conglomerates,  but  exposures  do  occui',  and 
they  are  usually  of  considerable  areal  extent,  well  cleared  off,  and  good. 
There  are  likewise  good  exposures  in  the  broad  area  underlain  by  the 
slates  to  the  south  and  southwest  of  Tower,  which  is  faii'ly  well  dissected 
by  stream  erosion.  This  statement  is  especially  true  of  areas  in  the 
immediate  vicinity  of  Pike  River  and  along  part  of  the  course  of  West 
Two  Rivers. 

Topography. — Considered  broadly,  the  Lower  Huronian  rocks  of  this 
area  occupy  relatively  low  ground,  the  higher  elevations  being  formed  by 
the  Archean  greenstones  and  the  iron-bearing  formation,  this  arrangement 
separating  the  Lower  Huronian  sediments  into  the  various  troughs  which 
have  already  been  described.  The  topography  of  the  areas  occupied  by 
the  Lower  Huronian  sediments  has  already  been  referred  to  (p.  36).  It  is 
fairly  rugged,  but  there  are  no  great  elevations.  The  rocks  have  been 
carved  into  a  series  of  north-northeast  to  south-southwest  trending,  rounded 
ridges  separated  by  valleys  occupied  by  swamps,  streams,  or  lakes. 

STRUCTURE. 

Considering  the  western  part  of  the  Vermilion  district  broadly,  it  will 
be  seen  that  the  Lower  Huronian  sediments  occupy  a  great  synclinorium, 
trending  N.  80°  E.,  with  Vermilion  Lake  lying  in  its  broadest  part,  and 
that  the    sediments    swing    around   the  anticline  of  greenstone    south  of 

rtGeol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  pp.  282,  525-538. 


THE  LOWER  HURONIAN.  279 

Tower  and  spread  out  southward  into  a  broad  area  in  which  exposures 
are  so  rare  that  no  structural  details  can  be  determined.  It  is  presumed, 
however,  that  this  is  also  a  great  synclinorium,  the  south  limb  of  which 
is  bounded  by  the  Giants  Range  granite,  here  beyond  the  limits  of  the 
map. 

Within  the  Vermilion  Lake  area  and  immediately  adjacent  to  it  ex- 
posures are  sufficient  to  enable  us  to  determine  some  of  the  details  of  the 
structural  features  of  the  sediments  in  them. 

In  this  part  of  the  Vermilion  district  it  willbe  noted  that  the  conglom- 
erate lies  upon  the  flanks  of  the  anticlinal  hills  formed  by  the  older  under- 
lying rocks.  Within  the  area  covered  by  the  Lower  Huronian  sediments 
alone,  the  Ogislike  cong-lomerate  occupies  the  anticlines,  for  example,  at  the 
Pike  Bay  oval,  which  is  an  anticlinal  area.  Ely  Island  is  made  up  chiefly 
of  the  Ogishke  conglomerate,  but  enough  of  the  adjacent  rocks  are  exposed 
to  show  plainly  the  structure.  The  conglomerate  occupies  the  main  central 
portion  of  the  island — in  fact,  nearly  all  of  the  western  two-thirds  of  the 
island — with  but  a  small  area  of  the  Knife  Lake  slates  flanking  it  on  the 
south.  In  the  eastern  part  of  the  island  the  conglomerate  is  intermixed  with 
eruptive  rocks,  the  granites  of  Vermilion  Lake,  from  which  it  is  derived  and 
with  which  it  is  intricately  infolded.  On  the  eastern  as  well  as  on  the 
western  end  of  the  island  the  conglomerates  are  coarser  near  the  center,  and 
grow  finer  and  finer  toward  the  sides.  This  change  is  most  noticeable  on 
the  south  side  of  the  island,  where  at  several  places  along  the  shore  the  Knife 
Lake  slates  grade  into  the  conglomerate  through  graywackes  of  intermediate 
grain. 

In  general  the  slates  occur  in  synclines  lying  between  anticlines  of 
older  and  harder  rocks,  and  ordinarily  these  synclines  coincide  with  the 
topographic  depressions.  In  some  places,  however — as,  for  instance,  north 
of  Tower,  between  the  west  end  of  Soudan  Hill  and  the  point  between 
Swede  and  Middle  bays — the  slates  occupy  a  minor  synclinorium  and  are 
extremely  plicated.  The  slates  of  this  particular  synclinorium  occupy  at 
this  place  higher  ground  than  the  adjacent  conglomerate  on  its  flanks, 
and  within  this  synclinorium  the  anticlines  of  slates  are  the  structural 
features  that  occur  at  the  greatest  elevation.  Structural  details,  such  as 
strike  and  dip,  were  observed  almost  exclusively  on  the  slates,  and  it  is 
consequently  by  a  study  of  the  slate  exposures  chiefly,  assisted  hj  observa- 


280  THE  VERMILION  IRON-BEARING  DISTRICT. 

tions  of  the  distribution  of  the  other  formations,  that  the  structure  of  the 
series  has  been  determined.  It  may  be  noted  here  that  in  the  absence  of 
any  striking  key  rocks  the  folds  in  the  slates  were  determined  chiefly  by 
the  distribution  of  the  slates  and  their  variation  in  strike  and  dip. 

The  general  strike  of  the  slate  beds  is  N.  80°  E.  The  slates  have 
been  very  closely  compressed,  and  consequently  many  of  the  exposures 
show  the  most  intricate  plications.  On  the  large  folds,  as  well  as  on  the 
plications,  the  strikes  extend  varyingly  to  nearly  every  point  of  the  compass, 
the  direction  depending  on  the  position  on  the  fold  of  the  place  where  the 
strike  is  taken.  The  dips  are  high  and  range  from  about  70°  S.  to  70°  N. 
The  northern  dip  is  the  more  coinmon  and  is  generally  not  far  from  80°. 
The  axes  of  the  folds  trend  approximately  N.  80°  E.,  and,  as  shown  by  the 
predominant  northward  dip  of  the  beds,  the  axial  plane  of  these  folds  is 
generall}'  overturned,  dipping  slightly  north.  In  addition  to  a  south- 
north  compression — to  be  exact,  the  pressure  came  from  a  direction  slightly 
west  of  north  and  east  of  south — producing  the  folds  trending  east  and 
west,  there  has  been  jwessure  at  right  angles  to  this,  which  caused  a 
corresponding  development  of  north-south  folds.  As  a  result  of  this  minor 
cross  folding,  the  axes  of  the  major  folds — the  folds  trending  east  and 
west — have  a  pitch  varying  from  90°  to  65°.  As  a  result  of  the 
compression,  sevefal  sets  of  joints  have  been  formed  in  the  rocks.  One 
trends  from  N.  60°  to  80°  E.,  in  close  agreement  with  the  strike  of  the 
bedding'  and  with  the  trend  of  the  axes  of  the  folds.  Another  set  is 
nearly  at  right  angles  to  the  above,  and  varies  from  north  and  south  to 
N.  10°  W.  The  joints,  however,  evidently  bear  definite  relations  to  the 
folds,  having  been  produced  by  the  same  forces  that  caused  the  folding,  for 
as  the  strike  of  the  beds  varies  upon  the  folds  the  joints  vary  also.  Thus 
on  the  j)oint  southeast  of  Sucker  Point,  where  the  beds  strike  N.  50°  W., 
the  joints  strike  N.  80°  W.  and  N.  30°  E.  The  close  compression  of  these 
slates  has  produced  a  fissility  which  is  verj-  uniform  throughout  the  region. 
It  is  verv  noticeable  in  the  slates,  and  its  general  strike  is  N.  80°  E., 
althougli  a  variation  of  a  few  degrees  to  south  or  north  can  be  found.  In 
general,  there  is  an  agreement  of  the  strike  of  tissilit}'  and  bedding,  but,  as 
Van  Hise  has  demonstrated,"  tlie  fissility  bears  diftereut  relations  to  the 

n  Principles  of  North  .American  pre-Cainbrian  geology,  by  C.  E.  Van  Hiae:  Sixteenth  Ann.  Eept; 
U.  S.  Geol.  Survey,  Pt.  I,  1896,  pp.  656-659. 


THE  LOWER  HURONIAN.  281 

beds  in  different  parts  of  the  folds.  Thus  on  the  Hmbs  of  the  folds  it  is 
essentially  parallel  with  'he  bedding,  whereas  at  the  ends  of  the  folds  it 
cuts  across  the  bedding  nearly  at  a  right  angle  to  it.  No  places  were  seen 
which  appeared  to  promise  a  supply  of  good  roofing  slate.  The  rocks  are 
very  g-enerally  much  broken  up  by  minor  joints,  but  at  considerable  depth 
possibly  rocks  might  be  found  in  such  condition  that  roofing  squares  could 
be  obtained. 

These  slates  are  everywhere  intersected  by  numerous  quartz  veins, 
especially  south  of  Pike  River  Bay.  The  presence  of  these  quartz  veins  in 
the  slates  gave  rise  to  the  rumor  of  the  occurrence  of  gold,  and  the  early 
history  of  the  Vermilion  district  is  the  history  of  attempts  to  obtain  gold 
from  veins  in  these  sediments — as,  for  instance,  at  Gold  Island,  near  the 
northern  part  of  the  lake. 

RELATIONS   OF  OGISHKE  CONGLOMERATE  AND   KNIFE  LAKE  SLATE. 

The  relations  of  the  rocks  to  one  another  are  so  clearly  shown  at  so 
many  places  that  it  is  scarcely  worth  while  to  discuss  them.  There  is  a 
great  conglomerate  normally  overlain  by  and  grading  up  into  a  great 
thickness  of  slate  through  the  intermediate  graywackes.  In  the  con- 
glomerates occur  masses  of  slate,  and  in  the  slate  likewise  occur  masses  of 
conglomerate.  EAadently  the  series  is  a  geologic  unit  which  is  divisible  into 
two  parts,  the  conglomerates  and  slates,  only  by  an  arbitrary  line  below 
which  the  conglomerate  predominates  on  the  whole,  and  above  which  the 
slates  predominate.  In  an  article  on  the  Vermilion  area  Smyth  and 
Finlay"  described  the  slates  as  the  oldest  rocks  of  the  area,  instead  of 
nearly  the  yoimgest. 

RELATIONS  TO   ADJACENT   FORMATIONS. 

Relations  to  Archean. — Where  the  series  is  in  contact  with  the  Ely 
greenstone  and  the  Soudan  formation,  the  conglomerate  normally  lies  next 
to  these  rocks,  and  consists  to  a  great  extent  of  pebbles  derived  from  them. 
Hence,  having  been  derived  from  them,  it  must  overlie  them  stratigraphic- 
ally  and  is  therefore  younger  than  they. 

Occasionally  the  conglomerate  occurs  in  very  thin  belts,  too  narrow  to 


«The  geological  structure  of  the  western  part  of  the  Vermilion  range,  Minnesota,  by  H.  L.  Smyth 
and  J   Ralph  Finlay:  Trans.  Am.  Inst.  Min.  Eng.,  Vol.  XXV,  1895,  p.  602. 


282 


THE  VERMILION  IRON-BEARING  DISTRICT. 


be  shown  on  the  map  without  great  exaggeration.  Sometimes  the  con- 
glomerate is  practically  wanting,  and  then  the  slates  abut  against  the 
Archean,  although,  as  a  rule,  actual  contacts   of  the   slates  and  adjacent 


Knife    LaUe   slates  Ogishke  conglomerate  Granite       Granite-porphyry^ 


twi*h  observed    (without  observed     (w It h  ob se r ved   (without  observed 
strike  and  di  p)      strike  and  d  ;  p)        stri  kfe  and  d  i  p)       strike  and  d  I  p) 


Fig.  18.— Detail  map  of  the  east  end  of  Ely  Islatid,  Vermilion  Lake,  Minnesota,  showing  actual  roeli  o.^posures,  by  J.  Morgan 

Clements  and  C.  K.  Leith,  1899. 

formations  are  wanting,  erosion  along  the    contact    having  removed  the 
slates,  which  are  softer  than  the  other  rocks  at  these  places. 

The  relation  of  the  sediments  to  the  granites  of  Vermilion  Lake  is 
exactly  the  same  as  their  relation  to  the  greenstone  and  the  iron  formation. 
The  basal  conglomerate  lies  next  to  these  eruptives  normally  and  consists 
chiefl}^  of  pebbles,  which  can   be  identitied  with  the  rocks  immediately 


THE  LOWER  HURONIAN.  283 

adjacent.  Moreover,  in  places,  as  we  get  farther  from  the  eruptives,  the 
gradation  from  the  conglomerate  to  finer-grained  sediments  can  be  dis- 
tinctly traced,  showing  plainly  that  the  conglomerates  were  derived  from 
the  eruptives,  and  that  consequently  they  are  of  later  age.  The  conglom- 
erates and  eruptives  especially  have  been  very  closely  infolded  and  they 
now  show  the  most  intricate  surface  relations.  T'he  large-scale  detailed 
maps  of  the  east  end  of  Ely  Island  (fig.  18),  and  of  the  point  south  of  Mud 
Creek  Bay  (Sheet  XXV  of  Atlas),  will  give  some  idea  of  the  intricacy 
of  their  surface  relations  and-  will  indicate  the  difficulties  of  successfully 
determining  the  geologic  structure.  This  intricacy  of  relationship  between 
the  sediments  and  eruptives  is  in  some  places  most  puzzling.  A  sketch, 
fig.  19  (p.  290),  made  in  the  field,  illustrates  the  relations  between  these 
two  rocks  which  were  seen  on  an  exposure  on  the  north  side  of  Ely  Island, 
near  the  east  end,  and  which  will  be  described  in  some  detail  further  on. 

The  character  of  the  conglomerate  is  usually  well  marked,  especially 
when  jasper  pebbles  are  abundant  in  it.  Under  such  circumstances  the 
pebbles  of  the  acid  igneous  rocks  in  the  conglomerate  can  also  be  identi- 
fied with  the  adjacent  masses  of  granite  and  porphyry.  But  where  both 
the  igneous  rocks  and  the  conglomerate  derived  from  them  have  been 
very  much  mashed,  and  especially  where  there  is  a  compai-atively  fine- 
grained sediment — in  other  words,  a  grit — it  is  very  difficult  to  discrim- 
inate between  them,  for  both  the  igneous  rocks  and  the  grits  derived  from 
them  have  been  sheared  into  white  to  gray  fissile  sericitic  schists  which 
have  practically  the  same  appearance.  It  is  not  improbable  that  some 
mistakes  have  been  made  on  the  detailed  maps  in  this  discrimination,  but 
extreme  care  has  been  exercised,  so  that  the  mistakes  are  unquestionably 
few,  and  while  they  may  aff"ect  the  determination  of  the  areal  distribution 
of  these  rocks  they  are  not  of  such  character  as  to  affect  the  interpretation 
of  their  general  relations. 

Relations  to  the  Giants  Range  granite. — South  of  Tower,  in  the  vicinity 
of  milepost  92,  on  the  Duluth  and  Iron  Range  Railroad,  the  Knife  Lake 
slates  are  intruded  and  metamorphosed  by  a  number  of  granite  dikes  which 
have  been  coiTclated  with  the  Griants  Range  granite.  The  fact  that  this 
granite  is  younger  than  the  Lower  Huronian  sediments  is  thus  clearly  shown. 

Relations  to  basic  dikes. — The  Lower  Huronian  sediments,  both  the 
Ogishke  conglomerate  and  the  Knife  Lake  slates,  are  cut  by  occasional  dikes 


284  THE  VERMILION  IRON-BEARING  DISTRICT. 

of  basic  rocks,  which  are  thus  }"Ounger  than  the  sediments.  Splendid 
examples  of  such  dikes  can  be  seen  on  Stuntz  Island  and  elswhere.  Some 
of  these  dikes  are  older  than,  and  some  of  them  are  essentially  contem- 
poraneous with,  the  Keweenawan  rocks. 

AGE. 

The  above  facts  concerning  the  relations  of  the  Ogishke  conglomerate 
and  the  Knife  Lake  slate  to  the  adjacent  formations  are  so  conclusive  that 
no  doubt  can  remain  as  to  the  relative  age  of  these  sediments.  They  lie 
immediately  on  the  Archean  Ely  greenstone  and  the  Soudan  formation, 
and  are  youngei-  than  these  and  than  the  granites  of  Vermilion  Lake  which 
penetrate  these  two  latter  formations.  They  are  older  than  the  Giants 
Range  granite  and  than  certain  basic  dikes  that  cut  them.  Since  the  sedi- 
ments lie  immediately  upon  the  Archean  and  are  overlain  by  another  series 
of  clastic  rocks,  as  has  been  found  from  the  study  of  the  contemporaneous 
rocks  in  the  Knife  Lake  area,  they  are  here  placed  at  the  base  of  the 
Alffonkian,  and  are  correlated  with  tlie  Lower  Huronian  series  of  the  other 
iron-bearing  districts  of  Lake  Superior. 

OGISHKE  CONGLOMERATE. 

This  conglomerate  was  first  so  called  because  it  is  well  developed  on 
and  near  Ogishke  Muncie  Lake,  and  the  use  of  the  term  has  been  continued 
on  account  of  its  appropriateness  and  becaiise  it  was  used  in  the  early 
literature  of  the  Vermilion  district. 

Atteiition  is  again  called  to  the  statement  already  made,  that  the  con- 
glomerate in  some  places  differs  somewhat  petrographically  from  that  of 
the  typical  area,  and  that  this  variant  phase  has  occasionally  been  called  by 
the  local  name  of  Stuntz  conglomerate.     (See  p.  278.) 

PETROGRAPHIC  CHARACTERS. 

Macroscopic  characters. — The  conglomerates  in  this  western  area  all 
possess  a  strong  family  resemblance.  On  the  weathered  surface  the  difterent 
beds  are  white  or  grayish  in  color.  This  light  color  is  due  to  pebbles  of 
rhyolite-porphyry,  microgranite,  granite,  and  granite-porphyr}',  which,  as  a 
rule,  have  very  light-colored  weathered  surfaces  and  are  the  main  constit- 
uents of  the  conglomerates.  In  a  few  places  there  is  a  good  deal  of  jasper 
pi'esent  in  angular  fragments  of  various  sizes.     Greenstone  fragments  are 


THE  LOWER  HURONIAN.  285 

found  occasional!}-,  but  they  are  much  rarer  than  one  would  expect  them  to 
be.  In  addition  to  the  kinds  of  rock  already  enumerated,  pebbles  of  black 
and  gray  cliert  and  yellowish-green  sericite-schists  were  observed. 

The  conglomerates  differ  locally  in  degree  of  coarseness,  varying  from 
coarse-grained  conglomerates,  with  bowlders  reaching  2  feet  in  diameter, 
to  those  in  which  the  majority  of  pebbles  are  about  4  to  5  inches  in 
diameter.  This  latter  facies  is  the  commoner.  Associated  with  these 
conglomerates  there  are  of  course  considerable  quantities  of  much  finer- 
grained  rocks,  which  would  naturally  be  called  consolidated  grits  or  gray- 
wackes,  but  which  are  here  mapped  with  the  conglomerates.  With  these 
are  likewise  occasionally  areas  of  slate.  On  the  map  an  attempt  has  been 
made  to  discriminate,  by  means  of  the  colors,  between  the  conglomerate 
and  the  slate,  but  a  close  examination  in  the  field  would  reveal  the  fact 
that  in  some  of  the  areas  marked  as  conglomerate  there  are  in  j^laces 
considerable  quantities  of  graywackes  and  slates  associated  with  and  lying 
in  the  midst  of  the  conglomerate.  The  areas  of  these  rocks  are  so  small 
in  proportion  to  the  area  of  the  conglomerates  that  no  attempt  has  been 
made  to  show  them  on  the  small  scale  maps  published  herewith. 

It  is  interesting  to  note  the  dependence  of  the  petrographic  character 
of  the  conglomerate  upon  the  adjacent  rocks  from  which  it  was  derived. 
Where,  for  example,  it  lies  next  to  a  certain  characteristic  porphyry, 
the  major  portion  of  the  conglomerate  is  formed  of  pebbles  and  fine  detritus 
of  the  porphyry.  On  the  other  hand,  where  the  conglomerate  lies  near 
the  iron-bearing'  formation,  fragments  of  this  formation  become  numerous, 
although  ordinarily  they  are  scarce.  The  pebbles  and  bowlders  in  the 
conglomerate  are  crossed  by  fracture  lines  which  divide  the  individual 
pebbles  in  it  into  more  or  less  rhomboidal  fragments.  This  fracturing  of 
the  fragments  and  the  occurrence  of  the  pieces  essentially  in  place  shows 
that  the  dynamic  action  that  produced  the  fracturing  took  place  after  the 
formation  of  the  conglomerate  and  that  only  slight  displacements -occurred 
as  the  result  thereof. 

OEIGIN    OF   THE    CONGLOMERATES. 

When  we  study  these  conglomerates  in  the  field  and  find  that  they 
are  made  up  of  pebbles  of  various  kinds  of  rock  lying  in  a  fine-grained 
clastic  matrix,  the  pebbles  of  a  certain  kind  of  rock  being  most  numerous 


286  THE  VERMILION  IRON-BEARING  DISTRICT. 

near  an  underlying  mass  of  the  same  kind,  and  when,  moreover,  we  find 
beds  of  grits  and  slate  alternating  with  the  conglomerate,  all  of  these 
showing  every  gradation  into  one  another  and  possessing  both  true  bedding 
and  false  bedding,  the  only  satisfactory  conclusion  we  can  form  concerning 
the  origin  of  these  rocks  is  that  they  are  true  clastic  conglomerates  of 
sedimentary  origin.  This  mode  of  origin  seems  so  obvious  that  its 
statement  appears  almost  uncalled  for,  and  it  is  made  only  for  the  reason 
that  a  strong  argument  has  been  made  by  previous  students  of  the  rocks 
of  this  area  for  the  brecciated  origin  of  these  conglomerates."  Reference 
has  already  been  made  in  previous  pages  to  the  formation  of  pseudo-con- 
glomeratic rocks  from  the  granites  of  Vermilion  Lake  by  dynamic  processes. 
The  first  description  of  these  pseudo-conglomerates  (friction  conglomerates) 
and  the  correct  explanation  of  their  origin  was  given  by  Smyth  and 
Finlay  in  the  article .  above  referred  to.  They  made  the  error,  however, 
of  attributing  this  method  of  formation  to  all  of  the  conglomeratic-looking 
rocks  of  that  part  of  the  lake  and  adjacent  shores  which  they  studied, 
including  great  masses  of  true  normal  conglomerates  occurring  in  great 
abundance  on  Stuntz  Island,  Stuntz  Bay,  and  elsewhere.  These  rocks,  it 
is  true,  are  intimately  associated  with  the  pseudo-conglomerates,  but  in 
most  places  may  be  readily  separated  from  them.  That  the  true  conglom- 
erates were  unquestionably  included  with  the  pseudo-conglomerates  is 
shown  by  the  fact  that  reference  was  made  to  the  conglomerates  occurring 
on  Stuntz  Bay  and  Island  as  examples  of  pseudo-conglomerates,''  whereas 
in  reality  they  are  typical  sedimentary  conglomerates  in  which  may  be 
observed  the  characters  referred  to  above  as  proving  indisputably  their 
sedimentary  origin. 

THICKNESS. 

The  bedding  in  the  coarse  conglomerate  is  poor,  but  grows  more 
distinct  as  the  grain  gets  finer  until,  as  in  the  rocks  here  called  graywackes, 
it  becomes  very  distinct.  It  is,  however,  generally  so  obscure  that  it  has 
not  been  possible  to  determine  it  with  great  accuracy  and  frequency. 
Moreover,  the  rocks  have  been  extensively  folded,  and  considerable  redupH- 
cation — which  it  has  not  been  practicable  to  determine — may  have  taken 


"Geological  structure  of  the  Vermilion  range,  by  H.  L.  Smyth  and  J.  Ralph  Finlay:  Ti-ans.  Am. 
Inst.  Min.  Eng.,  Vol.  XXV,  1895,  pp.  610,  629. 
''Op.  cit.,  p.  612. 


THE  LOWER  HURONIAN.  287 

place,  and  would  vitiate  any  estimates.  For  these  reasons  it  has  been 
found  impossible  to  determine,  even  approximately,  the  thickness  of  the 
conglomerate.  In  places  it  is  wanting  or  is  represented  merely  by  a  thin 
mass.  In  other  places,  as,  for  instance,  on  Vermilion  Lake,  it  shows  great 
development  and  must  be  very  thick. 

INTERESTING   LOCALITIES. 

The  islands  in  Pike  Bay  offer  good  exposures  of  the  typical  Ogishke 
conglomerate  of  the  western  area.  There  are  also  splendid  exposures  on 
the  large  island  in  sec.  14,  T.  62  N.,  R.  15  W.,  and  on  both  sides  of  Arm- 
strong Bay.  Smaller  exposures  occur  nearer  Tower,  one  southwest  of 
Tower  in  SW.  ^  of  sec.  6,  T.  61  N.,  R.  15  W.,  another  just  on  the  outskirts 
of  Tower,  on  the  south  slo23e  of  Lee  Hill,  and  another  on  the  south  slope  of 
Soudan  Hill. 

One  of  the  best  places  in  which  to  study  this  conglomerate  in  its  typical 
development  is  on  the  southwest  side  of  Stuntz  Island,  which  lies  across  the 
mouth  of  Stuntz  Bay  of  Vermilion  Lake.  On  the  bare  exposures  here  the 
conglomerate  is  made  up  of  pebbles  and  bowlders  of  granite-porphyi-y,  por- 
phyritic  microgranite,  rhyolite-porphyry,  a  feldspathic  porphyry,  jasper,  and 
comparatively  rare  fragments  of  greenstone.  The  coarsest  conglomerate 
lies  near  the  center  of  the  island  and  is  separated  from  the  acid  intrusives 
to  the  north  by  a  marked  depression.  Pebbles  of  the  intrusives  are  present 
in  the  conglomerate.  As  we  go  southward  across  the  exposures  the  con- 
glomerate grows  finer  and  occasional  beds  of  grit,  striking  east  and  west, 
occur  in  it  until  finally  on  the  extreme  southwestern  point  of  the  island  there 
may  be  seen  at  low  water  a  few  feet  of  typical  Knife  Lake  slates.  The 
evidence  here  is  conclusive  that  the  conglomerate  has  been  derived  from 
the  sediments  to  the  north  and  that  there  is  a  gradation  from  it  into  the 
Knife  Lake  slates  to  the  south.  On  the  highest  knob  at  the  west  end  of  the 
island  the  conglomerate  is  penetrated  by  a  number  of  basic  dikes  varying 
from  IJ  inches  to  6  feet  in  width.  At  one  place  near  the  highest  point  nine 
dikes  were  counted  within  a  distance  of  60  feet,  lying  essentially  parallel 
and  trending  a  little  south  of  east.  These  dikes  cut  across  the  schistosity 
and  the  bedding  of  the  conglomerate  and  also  across  the  fragments  in  it, 
showing  sharp  contacts.  They  do  not  seem  to  have  produced  any  contact 
effect  on  the  conglomerate.  The  dikes  themselves  are  only  very  slightly 
schistose,  and  the  schistosity  is  confined  to  the  edges. 


288  THE  VERMILION  IRON-BEARING  DISTRICT. 

On  the  hill  just  south  of  Mud  Creek  Bay  the  conglomerate  is  exposed 
at  a  number  of  places.  As  a  result  of  the  close  folding-  it  appears  in  very- 
complex  relationship  with  different  rocks,  namely,  the  Ely  ellipsoidal 
greenstone  and  the  various  granitic  rocks  of  Vermilion  Lake.  After  a 
careful  study  of  the  exceedingly  intricate  contacts  between  the  porphyries 
and  the  conglomerates,  which  at  first  led  to  the  belief  that  the  porphyries 
were  intrusive  in  the  conglomerates,  it  was  found  that  this  relationship 
was  due  to  the  close  infolding  of  the  rocks,  giving  zigzag  and  most  in-egular 
contacts.  This  relationship,  as  has  already  been  stated  in  pre"vious  pages, 
was  proved  by  the  identification  of  the  porphyrj*  pebbles  in  the  conglom- 
erate with  the  adjacent  porphyries.  The  detailed  map,  Sheet  XXY  of  the 
atlas,  shows  the  areal  distribution  of  these  rocks  on  this  point  and  will  give 
some  idea  of  the  intricacy  of  the  distribution. 

Reference  has  already  been  made  to  the  rocks  occurring  on  Ely  Island. 
A  good  place  at  which  to  study  the  close  relationship  of  these  rocks  is 
the  east  end.  Here  there  is  a  most  intricately  folded  complex  of  moderately 
fine-grained  granite-porphyry,  conglomerate,  and  graywacke.  The  distribu- 
tion of  these  is  shown  on  the  detailed  map  forming  fig.  18.  The  graywacke 
and  porphyry  when  looked  at  casually  resemble  each  other  very  strongly, 
but  when  examined  closely  they  can  readily  be  distinguished.  The  por- 
phyry is  studded  with  small  phenocrysts  of  quartz  and,  as  a  result  of 
weathering,  develops  an  exceedingly  rough  surface  in  detail,  something  like  a 
nutmeg  grater.  The  graywacke  contains  grains  of  quartz  which  in  many 
cases,  and  probably  in  most  cases,  are  phenocrj^sts  derived  from  the 
porphyry  and  in  some  instances  are  very  slightly  worn.  This  graywacke 
weathers  in  general  with  a  smooth  surface,  and  this  difference  in  the 
weathering  alone  will  usually  enable  one  to  distinguish  the  two  kinds  of 
rocks.  Where  the  gi-aywacke  is  in  very  massive  exposures,  and  especially 
where  the  graywacke  and  porphyry  have  both  been  sheared,  it  is  at  times 
extremely  difficult  to  separate  them.  As  the  result  of  the  shearing  and 
subsequent  weathering  both  the  porphyry  and  the  graywacke  are  likely  to 
develop  a  series  of  small  parallel  ridges  or  corrugations  on  the  surface. 
This  corrugated  way  of  weathering  was  not  so  noticeable,  however,  on  the 
por])h}Ty  as  on  the  graywacke.  At  this  place  the  infolding  of  the  porphyry 
and  the  sediments  is  exceedingly  complex.  We  find  fingers  of  the  one 
interlocked  with  fingers  of  the  other,  so  that  the  contact  forms  a  zigzag 


THE  LOWER  HURONIAN.  289 

line,  each  finger  pointing  to  a  small  fold — anticline  or  syncline.  The  plane 
of  contact  between  the  porphyry  and  the  sediments  varies  greatly.  In 
most  cases  the  porphyry  is  below  the  sediments,  but  in  some  cases  the 
fold  is  clearly  overtm-ned,  so  that  now  the  conglomerate  frequently 
lies  under  the  porphyry.  Between  these  extremes  any  position  of  this 
contact  plane,  from  flat  to  vertical,  may  be  seen.  Tliis  irregularity 
in  the  position  of  the  plane  caused  considerable  confusion  at  first  in 
the  determination  of  the  relationship  between  the  rocks.  For  some  time 
it  was  thought  that  the  porphyry  was  intrusive  in  the  sediments.  How- 
ever, further  study  showed  that  the  conglomerate  was  clearly  derived 
from  the  porphj-ry  and  that  the  relationship  mentioned  was  due  to  close 
folding.  A  further  factor  which  led  to  confusion  was  that  the  porphjT.y 
itself  simulated  somewhat  a  conglomerate,  for  it  is  marked  by  two  series 
of  fracture  lines  lying  close  together  and  crossing  each  other  at  such  an 
angle  as  to  produce  small  rhomboidal  blocks.  Further  shearing  took  place 
along  these  planes  of  parting,  and  eventually  the  angles  of  the  fragments 
were  more  or  less  completely  rounded,  and  the  areas  between  the  sub- 
angular  fragments  were  filled  with  schistose  material.  On  exposed  sur- 
faces the  massive  unfractured  parts  of  such  rocks  weather  less  readily 
than  do  the  schistose  portions  lying  between  them,  and  consequently 
stand  out  as  small  rounded  2Drojecting  areas  verj^  similar  to  the  pebbles 
in  a  conglomerate,  whicli  project  above  the  surrounding  matrix.  Closer 
examination  of  such  surfaces,  however,  shows  that  the  fragments  are  all 
of  one  kind  of  rock  and  that  the  apparent  matrix  lying  between  them  is 
but  sheared  material  of  essentially  the  same  nature  as  the  massive  portion. 
This  is  the  most  obvious  fact  noticed  in  a  study  of  them  and  enables  one 
readily  to  separate  such  fractured  and  sheared  porphyries  from  the  true 
conglomerates  derived  from  them,  which  are  made  up  invariably  of  frag- 
ments of  different  kinds  of  porphyries,  with  more  or  less  abundant  jasper 
fragments  and  an  occasional  fragment  of  greenstone.  The  strike  of  the 
axes  of  the  main  folds  at  this  locality  is  about  N.  80°  E.,  showing  that 
the  force  that  produced  the  folding  was  exerted  along  a  line  extending 
approximately  north  and  south.  As  the  result  of  this  compression  schistosity 
has  been  developed  in  the  sediments  and  in  the  underlying  intrusives. 
This  schistosit}'-  cuts  directly  across  the  minor  folds  shown  in  the  zigzag 
contacts  above  described  and  continues  from  the  sediments  into  and 
MON  XLV — 03 19 


290 


THE  VERMILION  IRON-BEARING  DISTRICT. 


through  the  igneous  rocks.     The  schistosity,  which  strikes  about  N.  70°  E., 
cuts  at  a  sharp  angle  the  sedimentary  bedding,  which  varies  from  N.  80°  E. 


•aiw?^ 


J5» 

_   -/ 


?^ 


^■^ 


**v 


>, 


'■^y^t^S^'^^'^v^'^^'^^' 


Fig.  19,— Sketch  showing  intricate  relationship  of  granite-porphyry  and  overlying  Ogishke  conglomerate  on  Ely  Island, 

Vermilion  Lake, 

to  east  and  west.     The  dip  of  the  bedding  varies  shghtly  from  75°  to  the 
north  to  vertical.     Indeed,  it  is  occasionally  found  with  a  dij)  of  85°  and 


THE  LOWER  HURONIAN.  291 

even  80°  to  the  south,  although  the  steep  northward  dip  is  the  one  which 
unquestionably  predominates.  The  sketch  reproduced  in  fig.  19,  which  was 
di-awn  to  scale  in  the  field,  illustrates  very  well  the  intricacy  of  the  contact 
between  the  sediments  and  the  igneous  rocks  and  shows  other  features  which 
at  first  tended  to  create  a  belief  that  the  porphyry  was  in  igneous  contact 
with  the  sediments.  The  exposure  reproduced  occurs  on  the  hill  at  the 
west  side  of  the  first  bay  west  of  the  northeast  point  of  Ely  Island  and  on 
the  north  shore  of  the  island.  Going-  along  the  contact  between  these  rocks, 
one  finds  the  contact  plane  lying-  at  diff"erent  angles,  but  on  this  particular 
exposure  the  folding  has  not  been  so  great  as  to  overturn  the  rocks  and  place 
the  conglomerate  under  the  igneous  rock.  Just  north  of  the  first  main  fold 
at  the  south  end  of  the  ex^josures  sketched  are  a  number  of  very  small 
flutings,  and  one  of  these  is  represented  as  it  occui-s  in  nature — as  connected 
with  the  main  mass  of  sediments  merely  by  a  small  neck.  A  little  farther 
north  of  this  place,  on  the  hill,  there  was  observed  a  small  mass  of  con- 
glomerate, rei^resented  in  the  sketch,  which  was  completely  separated  from 
the  sediment  and  sm-rounded  by  the  igneous  rock.  This  evidently  was 
closely  infolded  in  the  igneous  rock  and  afterwards  separated  by  erosion 
from  the  main  mass.  Here  the  process  of  separation  has  been  completed, 
whereas  in  the  mass  previously  described  erosion  had  gone  only  far  enough 
to  leave  merely  a  narrow  neck  connecting  it  with  the  main  area.  This 
isolated  area  of  conglomerate  appeared  much  like  an  inclusion  of  con- 
glomerate in  the  porphyi-}^,  and  was  so  construed  at  first,  but  later  more 
detailed  studies  showed  its  true  character  as  an  infolded  mass. 

On  the  east  side  of  Stuntz  Bay  of  Vermilion  Lake  the  conglomerate 
is  well  developed  and  is  exposed  over  large  areas  having  white  weathered 
surfaces.  Here,  as  at  the  other  places  noted,  the  conglomeratic  character 
is  plain,  the  fragments  being  well  rounded  and  ranging  from  minute  pebbles 
to  bowlders  2  feet  or  more  in  diameter.  At  one  place  there  is  a  coarse 
conglomerate  made  up  of  fairly  irregular  bowlders,  such  a  conglomerate  as 
is  often  deposited  near  a  shore  line  on  which  the  wave  action  has  not 
greatly  rounded  the  fragments.  Immediately  in  contact  with  this  coarse 
conglomerate  is  a  belt,  about  4  feet  thick,  of  beautiful,  regular  conglom- 
erate, such  as  would  be  produced  by  the  consolidation  of  a  shingle  beach. 
The  majority  of  the  pebbles  of  this  bed  vary  from  1  to  6  inches  in 
diameter.     The  conglomerates  have  not  been  very  much  metamorphosed. 


292  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  pebbles  have  been  frequently  crossed  by  fractures  which  divide  them 
into  rhomboidal  pieces,  but  have  not  in  general  been  greatly  deformed. 
The  fragments  usually  retain  their  relative  positions.  At  least  90  per  cent 
of  the  pebbles  in  the  conglomerate  are  rhyolite-  and  feldspar-porph^T}', 
granite- porphyry,  and  microgranite,  the  last  of  which  occurs  in  numerous 
exposures  in  the  immediate  vicinity.  Intermingled  with  these  in  very 
small  quantity  are  fragments  of  jasper  and  greenstone.  The  fragments  of 
porphyry  are  well-rounded  pebbles,  and  the  conglomerates  grade  into  the 
finer-grained  grits  and  slates  and  are  occasionall}-  traversed  by  bands  of 
this  finer-grained  material,  so  that  there  can  be  no  question  whatever  that 
they  are  normal  water-deposited  conglomerates. 

South  of  Tower,  near  milepost  92  on  the  Duluth  and  Iron  Range  Rail- 
road, there  is  a  cut  which  passes  through  the  Ogishke  conglomerate  and 
the  associated  Knife. Lake  slates.  The  conglomerate  is  very  Avell  exposed  on 
the  east  side  of  the  road,  where  the  weathered  surfaces  give  one  a  better 
oppoitunity  to  study  the  different  kinds  of  pebbles  in  the  rock  than  can  be 
had  in  the  small  fresh  exposm-es  in  the  cut.  The  conglomerate  is  of  essen- 
tially the  same  character  here  as  at  the  exposiu-es  near  Vermilion  Lake. 
The  Knife  Lake  slates  lie  north  and  south  of  it,  and  the  conglomerate  and 
the  overling  slates  have  been  intruded  by  both  acid  and  basic  dikes,  the 
acid  dikes  apparently  corresponding  to  the  Giants  Range  granite,  which 
forms  the  main  portion  of  the  Mesabi  or  Giants  range  bordering  the  northern 
portion  of  the  Mesabi  district.  This  occurrence  of  the  conglomerate  at 
this  place  is  e^^dently  due  to  a  subordinate  anticline  which  raised  it,  erosion 
having  then  removed  the  superimposed  slates  and  exposed  the  conglomerate 
as  we  now  find  it.  The  sediments  here  have  all  been  altered,  and  now  the 
matrix  of  the  conglomerate  and  the  finer-grained  bands  that  occur  occasion- 
ally in  it  have  been  metamorphosed  to  amphibole  and  especially  to  mica- 
schists,  whose  origin  could  not  be  determined  but  for  their  association 
with  and  gradation  into  undoubted  sediments. 

In  the  SW.  i  of  sec.  6,  T.  61  N.,  R.  15  W.,  at  the  second  falls  above 
the  bridge  on  the  county  road,  on  the  west  bank  of  West  Two  Rivers, 
there  is  a  cliff  consisting  of  Ogishke  conglomerate.  This  conglomerate  is 
here  only  a  short  distance  from  ellipsoidal  greenstone  of  the  Ely  formation, 
which  is  exposed  on  the  east  side  of  the  river,  and  it  consists  of  numerous 
pebbles  of  schistose  greenstone,   e\-idently   derived    from  the  underlying 


THE  LOWER  HURONIAN.  293 

Ely  greenstone,  as  well  as  predominant  pebbles  of  the  granite  rocks  of 
Vermilion  Lake.  This  is  the  point  nearest  to  the  basal  greenstone  at  which 
the  conglomerate  has  been  found  in  this  part  of  the  district. 

Another  basal  conglomerate  somewhat  similar  to  the  one  just  desci'ibed 
occurs  on  the  south  side  of  the  county  road  leading  from  Tower  to  Pike 
River,  about  325  paces  east  of  the  bridge  over  West  Two  Rivers.  This 
conglomerate  has  been  sheared  until  it  is  quite  schistose.  It  consists 
chiefly  of  fragments  of  greenstone,  jasper,  and  feldspar-porphyry. 

KNIFE  LAKE  SLATES. 

The  conglomerate  described  in  the  preceding  pages  is  overlain  by  the 
important  Knife  Lake  formation,  which  is  excellently  developed  upon  the 
shores  of  Vermilion  Lake  in  the  vicinity  of  Tower.  The  name  is  given 
to  the  formation  on  account  of  its  typical  development  near  Knife  Lake 
(p.  297). 

PETKOGEAPHIC  CHAEACTEES. 

It  has  been  stated  that  the  dividing-  line  di'awn  in  the  Lower 
Huronian  sediments  between  the  Ogishke  conglomerate  and  the  Knife 
Lake  slates  is  purely  arbitrary.  The  .transition  between  them  is  not 
sharp.  Among  the  conglomerates  there  are  a  few  interbedded  fine- 
grained sediments,  and  among  the  slates  there  are  a  few  fragmentals  that 
are  coarser  than  the  normal  slates,  and  show  gradations  between  the  slates 
and  the  conglomerates.  However,  the  slates  are  by  far  the  j)redominant 
kind  of  rock  in  the  areas  marked  on  the  accompanying  maps  with  the 
slate  color,  the  grits  playing  a  very  subordinate  r6le. 

Corresponding  to  differences  in  mineralogic  character  there  is  in  tlie 
slates  considerable  variation  in  color  and  texture.  The  normal  slates  are 
on  fresh  fracture  generally  a  slate  gray  to  dark-greenish  gray,  and  even 
light  greenish.  Sometimes  they  range  through  purplish  and  bluish-black 
rocks  to  a  dense  and  almost  black  slate.  They  usually  weather  with  a 
light-gray  to  brown  crust.  The  grain  of  the  slates  is  so  fine  that  one  can 
distinguish  no  individual  mineral,  unless  it  be  quartz,  except  in  the  phase 
that  approaches  the  grits.  The  banding  in  the  slates  is  caused  by  slight 
variations  in  the  quantity  of  the  minerals  of  different  color  constituting  the 
slates,  and  by  a  slight  difference  in  size  of  grain.     These  bands  within  the 


294  THE  VERMILION  IRON-BEARING  DISTRICT. 

slates  vary  in  thickness  from  a  fraction  of  an  inch  to  several  feet.  The 
bands  of  slate  themselves,  where  interlaminated  with  grits  and  near  the 
conglomerates,  also  vary  in  thickness  from  a  few  inches  up  to  30  paces. 
These  slate  beds  show  a  gradual  increase  in  thickness  as  they  occur  at  a 
greater  distance  from  the  conglomerates.  The  slates  are  in  places — as  in 
the  embayment  between  Tower  and  Lee  hills — very  heavily  impregnated 
with  pyrite,  which  is  scattered  through  them  in  cubes,  usually  altered 
more  or  less  to  limonite. 

Microscopic  examination  of  the  Knife  Lake  slates  and  associated  gray- 
wackes  shows  that  the  primary  constituents  are  feldspar,  quartz,  and  horn- 
blende in  fragments.  With  these  occui*  secondaiy  products — chlorite, 
epidote,  calcite,  sericite,  sphene,  and  pyrite.  In  the  coarse-grained  rocks 
the  various  constituents  can  readily  be  distinguished.  In  the  finer-grained 
ones  only  the  coarser  particles  can  be  clearly  recognized,  and  these  lie  in  a 
very  fine-grained  dark  matrix  whose  characters  can  not  be  positively 
determined,  but  which  probably  consists  of  'fine  dust  particles  derived 
fi-om  the  other  constituents,  with  which  may  occasionally  be  associated 
some  carbonaceous  material  (although  this  was  not  recognized  as  such) 
and  feiTUginous  matter,  the  last  being  the  chief  cause  of  the  dark  color. 

These  slates  vary  from  the  normal  slates  described,  which  prepon- 
derate, to  rocks  found  in  certain  portions  of  this  area,  which,  although 
showing  all  the  macroscopic  features  of  bedded  elastics,  nevertheless  under 
the  microscope  are  seen  to  have  been  recrystallized,  and  now  may  properly 
be  called  mica-  and  amphibole-schists  and  gneisses.  These  mica-  and 
amphibole-schists  and  gneisses  vary  from  light-gray  to  neai-ly  black  rocks. 
The  schists  have  a  light-brownish  weathering  crust.  One  can  disting'uish 
in  all  cases  in  them  the  mica  flakes,  the  amphibole,  and  very  frequently 
the  feldspar  and  quartz.  Difierences  in  color  and  size  of  grain  produce  a 
banding  in  these  metamorpliosed  rocks.  Usually  the  banding  stands  out 
much  more  plainly  on  their  weathered  surfaces  than  upon  the  fresh  fracture 
planes.  This  banding  in  the  schists  unquestionably  corresponds  to  lines  of 
original  bedding,  for  it  can  in  places  be  traced  uniuterruptedly  from  the 
slates  into  the  banded  mica-schists,  in  both  of  which  rocks  it  shows  the  same 
strike.  Moreover,  at  one  place  south  of  Tower,  on  exposui-es  east  of  the 
Duluth  and  Iron  Range  Railroad,  near  milepost  92,  one  niay  see  these 
schists  in  various  stages  of  formation,  and   on  these  schistose  rocks  there 


THE  LOWER  HURONIAN.  295 

are  still  present  most  perfect  examples  of  false  bedding  in  the  normal 
unmetamorphosed  slates. 

These  metamorphosed  slates  consist  of  green  hornblende,  actinolite, 
mica  (biotite,  muscovite,  and  sericite),  feldspar,  quartz,  chlorite,  rutile, 
ejjidote,  sphene,  apatite,  calcite,  garnet,  and  iron  oxide.  In  some  of  these 
the  garnet  and  muscovite  appear  as  porphyritic  constituents  full  of  inclu- 
sions of  tlie  other  minerals  of  the  rock,  thus  showing  that  their  origin  is 
later  than  that  of  these  constituents. 

THICKNESS. 

The  folding-  of  the  slates  has  resulted  in  excessive  crumpling  and  a 
slight  overturning  with  an  average  dip  of  about  80°  N.  That  this  condition 
exists  is  shown  by  a  number  of  minor  anticlines  and  synclines  which  have 
been  observed.  It  is  very  probable,  therefore,  that  the  thickness  of  the 
slates  has  been  several  times  repeated  in  the  area.  Bearing  the  above  facts 
in  mind,  one  will  readily  appreciate  the  statement  that  an  estimate  based 
on  the  width  of  the  slates  and  the  width  of  the  area  would  probably  give  a 
thickness  many  times  too  great.  As  such  an  estimate  would  only  lead  to 
erroneous  conclusions,  and  as  we  have  no  better  means  of  making  a  more 
accurate  estimate,  no  attempt  is  made  to  give  the  thickness. 

INTERESTING   LOCALITIES. 

The  general  characters  of  the  Knife  Lake  slates  occurring  in  the 
western  part  of  the  Vermilion  area  can  be  seen  at  many  places  on  the 
islands  in  Vermilion  Lake  and  on  the  shores  of  the  lake.  The  slates  are 
well  exposed  on  the  east  end  of  Sucker  Point  and  on  the  adjacent  shores 
of  the  mainland,  and  here  one  has  good  opportunities  to  examine  them  at 
localities  that  are  readily  accessible.  The  high  hills  east  and  southeast  of 
Swede  Bay,  in  the  SE.  ^  of  sec.  20,  T.  62  N.,  R.  15  W.,  afford  a  number 
of  bare  rounded  surface  exposures  of  these  slates,  and  here,  too,  the  results 
of  the  intricate  folding  to  which  they  have  been  subjected  can  be  studied. 
Similar  slates  may  be  observed  at  several  places  on  the  south  shore  of  Ely 
Island,  on  Canoe  Island,  and  on  the  island  south  of  its  eastern  end,  and  also 
on  the  south  shore  of  Pine  Island,  as  well  as  at  a  great  many  places  on  the 
lake.  These  slates  are  also  exposed  on  the  south  slope  of  Soudan  Hill,  just 
above  the  road,  and  on  the  road  at  the  crest  of  the  small  hill  in  the  town 


296  THE  VERMILION  IRON-BEARING  DISTRICT. 

of  Soudan.     Being'  near  the  base  of  the  formation,  the  slates  here  contain 
small  amounts  of  conglomerate  and  graywacke. 

More  interesting  than  these  common  phases  are  the  slates  which  occur 
in  the  southern  portion  of  the  area,  and  which  have  been  subjected  to 
nietamorphic  action  to  such  an  extent  that  they  have  been  transformed 
into  mica-  and  amphibole-schists.  Excellent  op^^ortunities  for  the  study 
of  these  metamorphosed  sediments  are  afforded  by  exposures  near  Pike 
River.  The  best  places  for  such  studv,  however,  are  in  the  cuts  along 
the  Duluth  and  Iron  Range  Railroad  between  Emban-ass  and  Tower, 
and  especially  in  those  between  East  Two  Rivers  and  milepost  92.  On 
these  exposures  the  sedimentary  character  of  the  rocks  is  clearly  shown  by 
sedimentary  banding,  false  bedding,  the  presence  of  large  bluish  frag-mental 
quartz  eyes  which  stud  some  of  the  beds,  and  in  the  vicinity  of  milepost 
92  by  the  fact  that  exactly  similar  sediments  are  there  interbedded  with  the 
Ogishke  conglomerate,  into  which  the  slates  grade.  Some  of  the  sediments 
have  been  so  extremely  metamorphosed,  however,  that  but  for  their  field 
relations  it  would  be  impossible  to  recognize  them  with  absolute  confidence 
as  derived  from  sediments.  It  should  be  noted  that  the  sediments  at  these 
exiDosnres  are  cut  by  granite  dikes,  and  that  the  change  in  the  sediments 
from  normal  slates  to  mica-  and  amphibole-schists  coincides  with  the 
appearance  of  the  dikes.  The  metamorphism  of  the  rocks  increases  south- 
ward alongr  the  railroad,  in  which  direction  the  dikes  become  more  numerous 
as  one  approaches  the  large  granite  areas  on  the  Giants  range,  from  which 
the  dikes  are  presumed  to  be  offshoots.  Winchell,"  who  has  noted  the 
metamorphism  of  the  graywacke  and  the  slates  to  mica-schists  south  and 
west  of  Tower,  attributes  this  metamorphism  to  the  Giants  Range  granite, 
but  classes  these  sediments  in  his  Keewatin  dixision.  The  sedimentary 
banding,  which  still  shows  very  plainly,  even  at  places  where  the  rocks 
have  been  metamorphosed  to  mica-schists,  is  evidently  indicative  of  a  differ- 
ence in  original  mineralogic,  and  hence  chemical,  composition.  In  spite  of 
the  metamorphism  these  original  differences  have  continued  to  exist,  and 
hence  the  sedimentary  banding  is  retained. 

oGeol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV,  1899,  p.  254,  and  PI.  LXVII 
anJ  LXXXVI. 


THE  LOWER  HURONIAN.  297 

KOT:FE  liAKE  AREA  OF  THE  LOWER  HUROJSriAN  SEDIMENTS. 

SUBDIVISIONS. 

The  Lower  Huronian  sediments  are  mucli  better  developed  and  more 
extensively  disti'ibuted  in  the  eastern  than  in  the  western  portion  of"  the 
Vermilion  district.  Knife  Lake,  a  prominent  topographic  feature  of  the 
eastern  part  of  the  district,  lies  in  the  sediments,  and  therefore  this  portion 
of  the  disti'ict  in  which  the  sediments  occur  will  be  called  the  Knife  Lake 
area. 

The  Lower  Huronian  sediments  of  the  Knife  Lake  area  may  be  con- 
veniently subdivided  into  (1)  the  basal  Ogishke  conglomerate,  (2)  the  Agawa 
formation  (iron  bearing),  lying  conformably  above  the  conglomerate,  and  (3) 
the  Knife  Lake  slates,  which  overlie  conformably  the  preceding  formations. 
Thus  it  will  be  seen  that  in  this  eastern  area  there  is  a  tripartite  divi- 
sion, whereas  in  the  Vermilion  Lake  or  western  area  the  Lower  Huronian 
could  be  subdivided  into  only  the  Ogishke  conglomerate,  and  the  Knife 
Lake  slates,  time  equivalents  of  the  Ogishke  conglomerate,  Agawa  forma- 
tion, and  the  Knife  Lake  slates  of  this  eastern  area.  The  intermediate  iron- 
bearing  Agawa  formation  of  the  Knife  Lake  area  has  no  known  stratigraphic 
equivalent  in  the  western  part  of  the  Vermilion  district. 

DISTRIBUTION,   EXPOSURES,   AND   TOPOGRAPHY. 

Distribution. — The  eastei'n  area  of  the  Lower  Huronian  sediments 
begins  a  few  miles  west  of  Ely,  in  sec.  4,  T.  62  N.,  R.  13  W.,  and  extends 
east  for  a  long  distance  beyond  the  international  boundary,  which  is  the 
eastern  limit  of  the  portion  of  the  district  included  in  the  accompany- 
ing map  (PI.  II).  Where  these  sediments  begin  at  the  west  the  area 
underlain  by  them  is  very  narrow,  and  this  tongue  continues  narrow  for 
several  miles  to  the  east,  gradually,  however,  widening  out.  Finally,  in 
the  vicinity  of  Moose  Lake,  the  continuation  of  this  narrow  belt  is  found  to 
■join  the  main  Lower  Huronian  sedimentary  area.  The  distribution  of  these 
rocks  for  this  part  of  the  area  will  be  mentioned  later.  To  the  south  of  the 
east-west  trending  area  above  mentioned  there  lies  a  narrow  belt  of  sedi- 
ments which  begins  on  Farm  Lake,  extending  about  east  and  west.  This 
belt  lies  along  both  sides  of  the  North  Branch  of  the  Kawishiwi  Rivei-, 
extending  southward  below  this  for  some  distance,  where  it  abuts  against 


298  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  iuti-usive  granite  and  continues  eastward  into  sec.  29,  T.  63  N.,  R.  10 
W.  The  continuation  of  these  sediments  to  the  east  is  also  inteiTupted 
by  the  intrusive  granite.  The  same  series  of  sedimentary  rocks  is 
again  found  east  of  the  above-mentioned  granite  in  sec.  20,  T.  63  N., 
R.  9  W.  From  here  they  have  been  traced  to  the  northeast,  where  they 
are  found  to  connect  with  and  to  form  part  of  the  same  \arge  area  south 
of  Moose  Lake  into  which  the  northerly  tongue  pre\'iously  mentioned 
merged.  In  this  portion  of  the  district,  that  is,  in  the  vicinity  of  Moose  Lake, 
the  Lower  Huronian  sediments  are  found  to  extend  over  the  greater  por- 
tion of  the  area  surveyed.  These  sediments  are,  however,  subdivided  into 
several  partly  disconnected  areas  or  tongues  by  intervening  areas  underlain 
by  Archean  rocks  as  well  as  by  intrusive  masses  of  acid  rocks  somewhat 
younger  than  the  sediments.  Continuing  our  observations  on  the  dis- 
tribution of  the  Lower  Huronian  sediments  from  the  area  west  of  Snow- 
bank Lake,  we  note  first  that  on  the  south  this  area  is  disconnected  from 
an  area  underlain  by  related  rocks  on  the  southeast  side  of  Snowbank 
Lake  by  the  intervening  Snowbank  granite.  This  belt,  however,  extends 
around  the  east  side  of  the  Snowbank  Lake  area  and  connects  on  the 
northeast  with  the  similar  sediments  which  sweep  around  the  northwest 
side  of  the  lake.  Where  these  join,  to  the  northeast  and  east  of  Snow- 
bank Lake,  they  underlie  an  area  which  has  very  nearly  the  same 
width  as  the  Vermilion  district.  This  main  mass  of  the  sediments 
continues  on  east  over  Ensign  and  Knife  lakes.  To  the  south  of  Knife 
Lake  the  main  area  underlain  by  the  sediments  is  interrupted  by  small 
areas  of  Archean  rocks  as  well  as  by  the  Cacaquabic  granite,  which  is 
younger  than  the  sediments.  To  the  east  of  Ogishke  Muncie  Lake  the 
Lower  Huronian  sediments  are  divided  into  two  main  belts  by  a  west- 
ward-projecting massive  of  Archean  greenstone  which  lies  immediately 
south  of  the  granite  of  Saganaga  Lake  and  in  juxtaposition  with  it.  These 
belts,  a  southern  and  a  northern  one,  can  be  traced  around  the  interrupting 
Archean  greenstone  and  granite  of  Saganaga  Lake  for  a  great  distance 
beyond  the  Canadian  border  to  the  north  of  this  separating  area.  On  the 
south  this  sedimentary  series  ends  just  east  of  Gobbemichigamma  Lake,  in 
sec.  30,  T.  65  N.,  R.  4  W. 

Exposures. — The  country  underlain  by  the  conglomerates  and  slates  is 
cut  up  by  numerous  lakes  and  is  for  the  most  part  bare  of  timber  of  large 


THE  LOWER  HURONIAN.  299 

growth,  partly  by  reason  of  extensive  forest  fires  and  partly  for  lack  of 
good  and  abundant  soil;  consequently  the  exposures  of  these  sediments  are 
numerous  and  usuallj^  of  exceptionally  large  size. 

Topography. — The  topography  of  the  Knife  Lake  area  of  the  Lower 
Huronian  sediments  is  very  rough,  although  the  features  as  a  whole  are  on  a 
comparatively  small  scale.  In  general  the  topography  in  this  area  is  much 
more  accentuated  than  it  is  in  the  area  underlain  by  the  same  sediments  in 
the  western  portion  of  the  district  already  described  (p.  278).  The  maxi- 
mum diiference  in  elevation  is  400  feet,  the  difference  between  the  level  of 
Ogishke  Muncie  Lake  and  the  adjacent  hills.  Reference  has  already  been 
made  (see  p.  45)  to  the  fact  that  the  lakes  in  this  part  of  the  district  are  rela- 
tively deep,  as  has  been  shown  by  the  few  soundings  taken.  The  maximum 
depth  found  in  a  lake  in  the  sediments  is  nearly  200  feet.  In  reality,  then, 
the  difference  between  lowest  valleys  and  lake  basins  and  highest  hills  is 
about  600  feet.  The  hills  and  ridges  have  the  usual  east-northeast-west- 
southwest  trend,  with  narrow,  deep  valleys  occupied  by  streams  and  lakes 
lying  between.  The  slates,  on  the  other  hand,  generally  form  the  lower 
hills.  These,  while  occasionally  rounded,  are  generally  more  or  less  angu- 
lar, more  nearly  coi-respoiiding  to  the  appearance  of  the  slate  hills  in  non- 
glaciated  territory,  although  by  no  means  so  angular  as  these. 

Normally  the  conglomerates  occupy  higher  levels  than  do  the  slates 
which  lie  next  to  them,  and  these  hills  of  conglomerate  have  fairly  well- 
rounded  contours.  In  portions  of  this  area,  however,  the  slates  are  very 
siliceous,  and  as  a  result  of  their  great  hardness  form  some  of  the  highest 
hills.  In  the  area  underlain  by  slates  sheer  cliffs  are  common,  some  of 
them  reaching  a  height  of  100  feet  above  the  lakes. 

The  topography  has  been  greatly  influenced  by  the  structure.  This 
will  be  referred  to  in  the  succeeding  paragraphs. 

STRUCTURE. 

The  Lower  Huronian  sediments,  from  the  westernmost  point  where 
they  are  found  (see  PI.  II),  just  west  of  Ely,  to  their  eastern  extension,  where 
they  abut  against  the  Saganaga  anticlinal  area,  occupy  a  sjmclinorium 
which  trends  approximately  K.  70°  E.  and  continues  around  the  northern 
side  of  the  Saganaga  anticlinal  area  into  Canada.  This  synclinorium  is 
narrow -in  its  western  part  and  widens  out  toward  the  east.     In  that  portion 


300  THE  VERMILION  IRON-BEARING  DISTRICT. 

of  the  district  where  the  sediments  have  a  considerable  width  their  conti- 
nuity is  intemipted  by  numerous  anticlines  of  older  rocks.  Here  and  there 
a  boss  of  granite  which  has  been  intruded  through  these  sediments  is  found, 
the  sediments  wrapping  around  it.  These  areas  also  are  anticlinal  in 
structure.  For  the  most  part  the  various  anticlinal  areas  are  commonly 
outlined  by  conglomerates  which  lie  on  the  flanks  of  anticlinal  hills  whose 
centers  are  occupied  by  an  older  rock.  Where  the  sediments  alone  occur  the 
conglomerates  occiipy  the  centers  of  the  anticlinal  areas.  The  bedding  is 
so  poorly  preserved  in  the  conglomerates  as  a  rule  that  one  can  not  get 
many  strike  and  dip  determinations  to  assist  in  interpreting  the  structure. 
Unquestionably  these  conglomerates  must  have  been  folded  with  the  other 
sediments,  although  such  folding  can  not  be  traced  in  detail  on  their 
exposm'es.  It  is  shown,  however,  by  the  distribution  of  the  slates,  which 
dip  away  from  such  anticlinal  areas  of  conglomerate.  The  slates  invariably 
occur  within  the  syncliues,  forming  depressions  as  a  result  of  their  initial 
position  at  the  bottom  of  the  syncline  and  as  a  result  of  the  relative  ease 
with  which  they  are  eroded.  This  is  shown,  for  example,  in  the  broad  area 
of  slate  surrounding  Knife  Lake.  Exceedingly  fine-grained,  verA^  cherty 
slates,  breaking  with  conchoidal  fracture,  lie  about  in  the  axis  of  Knife  Lake. 
As  we  go  farther  south  from  this  point  the  sediments  get  coarser,  graywackes 
gradually  becoming  associated  with  the  slates,  and  finally  the  sediments 
grade  into  conglomerates.  This  same  condition  exists  north  of  the  lake, 
although  there  the  conglomerates  are  not  so  greatly  developed  as  to  the  south 
of  it.  Within  this  and  other  broad  slate  areas  small  slate  anticlines  very 
probably  occur,  for  although  no  such  anticlines  have  been  clearly  demon- 
strated to  exist,  indications  of  them  have  been  found. 

As  is  shown  on  the  map,  this  broad  area,  underlain  by  the  Lower 
Hm-onian  sediments,  is  separated  from  several  detached  areas  to  the  south 
by  intervening  highlands,  occupied  by  the  Ely  greenstone  and  the  Snow- 
bank and  Cacaquabic  granites,  named  in  order  of  age.  In  the  area  south 
of  these  highlands,  formed  of  the  older  rocks,  the  structure  of  the  sediments 
is  totally  different  from  that  seen  in  the  large  area  to  the  north.  South  of 
these  anticlinal  highlands  the  sediments  occur  in  a  southward-dipping  mono- 
cline which  extends  with  few  interruptions  from  the  vicinity  of  Snowbank 
Lake  to  the  eastern  end  of  the  slate  area  on  Paul  Lake.     There  is  a  contin- 


THE  LOWER  HURONIAN.  301 

uous  narrow  monocline  of  slate  extending  from  Cacaquabic  Lake  east  to 
Lake  Gobbemichigamma,  and  lying  between  the  high  Twin  Peaks  anticline 
on  the  north  and  the  g-abbro  on  the  south.  This  belt  of  rocks  has  been 
much  metamorphosed  by  the  gabbro. 

The  slates  of  the  Lower  Huronian  show  the  effect  of  the  pressure  much 
better  than  do  the  conglomerates,  and  the  remainder  of  the  descri23tion  of 
the  structure  of  the  sediments  applies  specifically  to  the  slates  forming  the 
upper  part  of  the  series.  In  addition  to  the  close  folding,  which  is  indicative 
of  great  pressure,  the  Lower  Huronian  Knife  Lake  slates  have  been  jointed 
and  faulted,  and  schistosity  and  cleavage  have  been  produced.  In  general, 
the  major  joints  have  an  east-northeast  ti-end,  and  the  cross  joints  have  a  trend 
not  quite  at  right  angles  to  the  first.  These  joints  make  the  slates  break 
into  rhomboidal  blocks  and  are  the  chief  cause  of  the  formation  of  the  high 
clifi"s.  The  strike  of  the  joints  varies  with  the  direction  from  which  the 
pressure  producing  them  came.  Thus,  in  the  western  part  of  the  area, 
where  the  pressure  was  apparently  N.  10°  W.  to  S.  10°  E. — that  is,  perpen- 
dicularly to  the  axis  of  the  folds  and  to  the  strike  of  the  bedding — one  set  of 
joints  trends  about  N.  80°  E.,  and  another  trends  in  a  direction  very  nearly 
at  right  angles  to  it,  making  an  angle  a  little  less  than  a  right  angle  with 
the  first  set  of  joints.  In  the  eastern  part  of  the  district,  however,  where 
the  slates  abut  against  the  granite  of  Saganaga  Lake  and  wrap  around  it,  the 
direction  of  the  joints  changes.  Thus  in  sec.  35,  T.  66  N.,  R.  6  W.,  three  sets 
of  joints  were  noted.  The  first  set  strikes  N.  25°  E.,  and  dips  30°  to  the 
northwest,  corresponding  closely  with  the  strike  of  the  schistosity.  The 
second  strikes  N.  10°  W.  and  dips  85°  to  the  west.  This  agrees  with  the 
bedding.  The  third  set  strikes  N.  60°  E.  and  dips  85°  to  the  southeast. 
The  strike  of  these  joints  evidently  influences  very  materially  the  shape  of 
the  lakes  in  this  part  of  the  district.  For  instance,  in  the  case  of  the  lake 
in  sees.  34  and  35,  T.  66  N.,  R.  6  W.,  these  joints  can  be  seen  to  determine 
the  long  direction  of  the  lake  and  the  trend  of  the  bays. 

In  a  few  places  we  find  that  the  slates  show  minor  faulting  along  the 
joints.  No  cases  were  seen,  however,  where  the  throw  was  more  than 
about  1  foot.  South  of  Fox  Lake,  at  a  place  north  1,915  paces,  west  600 
paces  from' the  southeast  corner  of  sec.  35,  T.  65  N.,  R.  6  W.,  the  interbedded 
slates  and  graywackes  are  broken  and  slightly  faulted.  The  fault  plane 
runs  N.  10°  W.     The  shearing  accompanying  the  faulting  has  affected  a 


302  THE  VERMILION  IRON-BEARING  DISTRICT. 

zone  about  3  feet  wide.  In  the  midst  of  this  zone  there  is  a  horse  of  the 
country  rock.  Around  it  the  material  is  sheared  and  brecciated,  and  the 
infiltration  of  silica  has  taken  place  subsequent  to  this  shearing.  The 
beds  in  the  sediments  on  both  sides  of  the  fault  have  been  bent.  The 
amount  of  displacement  could  not  be  measured,  but  seems  to  have  been 
slight — a  ver}^  few  feet  at  the  most. 

The  schistosity  and  cleavage  which  have  been  produced  are  well 
marked  on  the  slates.  They  show  the  variable  relations  to  the  bedding 
planes  which  are  shown  by  Van  Hise  "  to  be  consequent  upon  their  mode 
of  fonnatiou,  and  are  clearly  the  result  of  the  compressive  forces  which 
caused  the  folding.  Thus  they  may  be  essentially  parallel  to  the  bedding 
on  the  flanks  of  the  folds,  and  vary  from  this  position  to  a  position  at  right 
angles  to  it,  near  the  apices  of  the  folds.  This  cleavage  can  be  well  seen 
on  the  good  exposures  southwest  and  west  of  the  portage  from  Moose  into 
Flask  Lake.  South  of  Ogishke  Muncie  Lake,  where  the  beds  strike  N.  25° 
to  30°  W.,  the  schistosity  strikes  N.  60°  E.  The  diff"erence  in  the  behavior 
of  the  soft  and  hard  beds — that  is,  the  weak  and  the  strong  beds — under 
the  same  condition  of  pressure  are  well  brought  out  at  one  place  upon 
Ogishke  Muncie  Lake.  At  the  southwest  end  of  the  long  point  projecting 
southwest  into  sec.  27,  T.  65  N.,  R.  6  W.,  at  the  southwest  end  of  Ogishke 
Muncie  Lake,  there  are  in  the  slate  near  the  water's  edge  alternating  bands 
of  harder  and  softer  materials.  In  the  softer  bands,  cleavage  running 
parallel  to  the  bedding  has  been  produced,  while  in  the  harder  ones  cross 
joints  have  been  formed,  i-unning  practically  perpendicular  to  the  cleavage 
in  the  soft  beds.  This  difference  is  evidently  due  to  the  difference  in  the 
elastic  strength  of  the  two  rocks.  The  one,  the  slate,  practically  flowed 
under  pressure,  while  the  other  was  fractured.  The  deformation  evidently 
took  place  while  these  rocks  were  in  the  zone  described  as  the  combined 
zone  of  flowage  and  fracture.' 

Excessive  crumpling  is  very  noticeable  in  the  cherty  layers  and  in  the 
slates.  This  crumpling  is  especially  well  shown  on  the  portage  between 
Fox  and  Agamok  lakes,  and  is  illustrated  in  fig.  1,  PI.  Y,  Minnesota 
Geological  Survey,  Vol.  IV.     The  bands  here  are  fractured  along  planes 

"Principles  of  North  American pre-Cambrian  geology,  bj'C.  R.  Van  Hise:  Sixteenth  Ann.  Rept. 
U.  S.  Oeol.  Survey,  Pt.  I,  1896,  pp.  .573-874. 
''Il)icl. 


THE- LOWER  HURONIAN.  303 

which,  make  angles  somewhat  less  than  a  right  angle  with  each  other.  The 
schistosity  in  the  softer  bauds,  joroduced  as  the  result  of  shearing,  is  nicely 
shown  in  places  on  these  sediments. 

RELATIONS. 
KEIxATIONS   OF   THE    SEDIMENTARY   MEMBERS    OF   THE    SERIES    TO    ONE    ANOTHER. 

The  relations  of  the  Ogishke  conglomerate,  the  iron-bearing  Agawa 
formation,  and  the  Knife  Lake  slates  to  one  another  is  that  of  tlxree 
conformable  formations,  with  the  Ogishke  conglomerate  at  the  base  and 
the  Knife  Lake  slate  at  the  top.  Thej  occur  constantly  in  this  position, 
the  iron-bearing  formation  being  wanting  at  some  places,  but  present  at 
others.  There  are  gradations  between  the  formations.  The  lines  which 
have  been  drawn  are  based  upon  the  petrographic  character  of  the  rocks 
and  the  preponderance  of  the  various  kinds. 

RELATIONS   OF    THE    LOWER   HURONIAN    SEDIMENTS    TO    THE    ADJACENT    FORMATIONS. 


RELATIONS    TO    THE    .\J!CHEAS. 


Relations  to  JEly  greenstone. — The  relations  of  the  Lower  Huronian 
sediments  to  the  Archean  greenstones  are  clearly  shown  at  a  number  of 
places  where  the  Ogishke  conglomerate  has  been  found  in  association  with 
them.  As  a  rule  the  conglomerate  lies  upon  the  flanks  of  the  greenstone 
anticlines  and  is  made  up  chiefly  or  solely  of  pebbles  and  bowlders  which 
can  be  identified  with  the  rocks  constituting  the  Archean  complex,  so  as  to 
show  unmistakably  their  source.  Thus,  for  example,  at  a  great  number  of 
places  south  of  Moose  Lake  the  conglomerate  was  observed  in  direct 
contact  with  the  greenstones,  which  occur  in  conspicuous  ridges  forming 
the  cores  of  the  anticlines.  In  some  places  the  conglomerate  lies  immedi- 
ately adjacent  to  the  fine-grained  ellipsoidal  greenstone,  and  at  other 
places,  where  the  ellipsoidal  portions  have  been  removed  by  erosion  from 
the  greenstone  mass,  the  conglomerate  lies  against  the  coarse-grained 
greenstone  which  normally  is  at  some  distance  from  the  border  of  the 
greenstone  areas.  Moreover,  wherever  finer-grained  forms  of  the  elastics 
showing  bedding  occur,  it  is  usually  found  that  this  bedding  is  essentially 
parallel  with  the  contact  of  the  sediments  and  the  underlying  greenstones. 

The  contact  of  the  Ogishke  conglomerate  with  the  greenstones  was 
also    observed  on   the  north   side   of   Twin  Peaks  ridge  and   the    occur- 


304  THE  VERMILION  IRON-BEARING  DISTRICT. 

rence  there  was  in  general  agreement  with  the  description  given  by 
N.  H.  Winchell."  Furthermore,  the  contact  was  found  between  the  con- 
glomerate and  the  small  ridge  of  greenstone  which  lies  just  along  the  south 
shore  of  Ogishke  Muncie  Lake,  and  a  number  of  additional  contacts  were 
observed  on  the  south  side  of  the  great  anticline  north  and  northeast  of 
Gobbemichigamma  Lake.  The  fragmental  character  of  some  of  these 
deposits  was  recognized  by  the  Minnesota  survey,  but  it  was  not  seen  that 
they  were  sedimentary  deposits  of  later  age  than  the  greenstone.  They 
were,  on  the  contrary,  regarded  as  fragmental  volcanic  rocks,  and  were 
included,  with  the  greenstone,  in  the  Archean.'' 

Relations  to  the  Soudan  formation. — The  actual  contact  between  the 
conglomerate  and  the  iron-bearing  formation  was  observed  at  only  one 
place.  Here,  however,  the  evidence  is  indisputably'  clear.  The  jasper  of 
the  Soudan  formation  is  overlain  by  a  conglomerate  containing  fragments 
of  jasper  derived  from  it  as  well  as  fragments  of  greenstone  derived  from 
the  greenstones,  which  in  their  turn  underlie  the  iron-bearing  formation. 
In  addition  to  this  direct  contact,  where  the  evidence  is  perfectly  clear, 
there  have  also  been  found  at  a  number  of  places  scattered  all  over  the 
district  quantities  of  jasper  pebbles  in  the  conglomerate.  Their  presence 
is  sufficient,  of  course,  to  prove  that  the  Soudan  formation  is  older  than 
these  conglomerates. 

Tlie  fragments  of  slate  and  of  the  conglomerate  or  breccia  which 
occur  in  the  Ogishke  conglomerate  south  of  Moose  Lake  are  of  especial 
interest,  since  they  indicate  the  existence  of  a  series  of  clastic  sediments 
prior  to  the  formation  of  the  Ogishke  conglomerate.  The  field  e-vadence 
for  such  a  clastic  deposit  below  the  normal  iron-bearing  formation  has 
already  been  given.  In  this  series  there  are  slaty  rocks  associated  with 
conglomeratic  elastics.  The  fragments  of  slate  and  conglomerate  may  very 
well  have  been  derived  from  these.  In  the  conglomerate  there  were  found 
a  number  of  slate  pebbles.  Their  source  has  not  been  very  satisfactorily 
accounted  for.  If  we  accept  the  presence  of  certain  sediments  mentioned 
as  lying  in  a  position  between  the  iron-bearing  formation  and  the  green- 
stone as  indicative  of  the  fragmental  sedimentary  horizon  underljnng  the 

"Geol.  and  Nat.  Hist.  Survej' of  Minnesota,  Fifteenth  Ann.  Rept,,  1886,  pp.  372-374.  Final 
Kept,  Vol.  IV,  1899,  p.  451. 

''Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV,  1899,  p.  466. 


THE  LOWER  HURONIAN.  305 

jaspers,  then  tliese  slates  are  accounted  for,  as  they  occur  in  conglomerates 
younger  than  and  containing  fragments  of  the  jasper.  In  a  similar  way 
the  conglomerate  or  breccia  pebbles  observed  can  be  accounted  for.  It 
should  be  noted,  however,  in  connection  with  the  explanation  of  the  source 
of  these  pebbles,  that  we  do  not  know  whether  they  are  true  conglomerates 
or  merely  friction  breccias,  or  pseudo-conglomerates.  In  the  greenstone 
south  of  Moose  Lake  there  were  numerous  small  zones  which  had  been  so 
extremely  fractured  and  then  after  the  fracturing  had  been  so  sheared  that 
in  ma;ny  cases  friction  breccias  have  been  produced  which  closely  resemble 
normal  conglomerates  and  from  which  it  would  be  perfectly  possible  to 
derive  the  pebbles  seen  in  the  overlying  conglomerates  were  the  breccias 
produced  before  the  sediments  were  formed. 

Belations  to  the  granite  of  Saganaga  Lake. — In  the  northeastern  part  of 
the  district  the  Ogishke  conglomerate  is  very  close  to  the  granite  of 
Saganaga  Lake  and  in  several  places  contacts  between  these  two  rocks 
have  been  found  and  their  relationships  thereby  made  perfectly  clear.  In 
several  places  a  great  bowlder  conglomerate  has  been  found  immediately 
overlying  the  granite  of  Saganaga  Lake  and  consisting  essentially  of  frag- 
ments of  this  granite.  Detailed  description  has  already  been  given  of  the 
field  relations  of  the  granite  of  Saganaga  Lake  to  this  conglomerate  under 
the  description  of  the  granite  (p.  271),  and  it  was  there  shown  that  the  idea 
held  b}"  Lawson  that  the  granite  of  Saganaga  Lake  was  intrusive  in  the 
conglomerate  was  untenable,"  hence  it  will  not  be  necessary  to  repeat  this 
description  here.  From  the  evidence  it  is  perfectly  clear  that  the  Ogishke 
conglomerate  is  young-er  than  the  granite  of  Saganaga  Lake. 

RELATIONS    TO    LOTVEK   HUKONIAN. 

Belations  to  the  Giants  Range,  Snowbank,  and  Cacaquabic  granites,  and 
various  dikes  of  granite  and  granite-porphyry. — In  the  western  portion  of 
the  Vermilion  district  there  is  found  a  conglomerate — correlated  with  the 
Ogishke  conglomerate — which  is  in  contact  with  the  Giants  Range  granite, 
and  has  been  penetrated  by  dikes  from  this  granite.  In  the  vicinity  of 
Snowbank  Lake  a  similar  conglomerate  practically  surrounds  the  area 
underlain  by  the  Snowbank  granite,  and  in  a  great  number  of  cases  it  has 
been  found  to  have  been  penetrated  by  dikes  from  this  granite.  A  portion 
of  the  area  underlain  by  the  Cacaquabic  granite  is  likewise  surrounded  by 

a  Lake  Superior  stratigraphy,  by  A.  C.  Lawson:  Am.  Geologist,  Vol.  Ill,  1889,  pp.  320-327. 
MON  XLV— 03 20 


306  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  Ogishke  conglomerate  and,  as  in  the  preceding  cases,  the  conglomerate 
is  found  to  be  penetrated  by  offshoots  from  this  granite  massive.  At  other 
places  in  the  district,  where  the  conglomerate  is  distant  from  the  granite 
massive,  it  is  cut  by  dikes  of  rhyolite-porphyries  or  granite-porphyries,  or 
possibly  of  both.  In  all  cases,  however,  the  relationship  is  clearly  that  of 
intrusion,  the  conglomerates  being  intruded  and  metamorphosed  by  the 
granites.  Hence  the  conclusion  that  the  conglomerates  are  of  greater  age 
than  the  granites  which  penetrate  them. 

Relations  to  certain  basic  and  intermediate  dikes  of  Lower  Huronian  age. — 
At  numerous  places  dikes  of  slightly  varying  character — altered  basalts 
and  lamprophyres — have  been  found  cutting  the  Lower  Huronian  rocks. 
None  of  these  dike  rocks  are  found  as  dikes  in  the  overlying  Upper 
Huronian  series.  Hence  they  are  believed  to  have  been  intruded  in  the 
Lower  Huronian  rocks  at  about  the  time  when  they  were  being  folded  and 
intruded  by  the  aforementioned  granites.  Certainly  some  of  the  dikes  are 
later  than  some  of  the  granites,  as  they  are  found  cutting  the  granites. 

REL.VTIONS   TO   THE   UPPER   HURONIAN   SERIES. 

It  was  found  that  the  most  difficult  problem  of  relation  to  be  solved' 
was  that  of  the  relationship  between  those  rocks  which  are  here  classed  as 
the  Lower  Huronian  (consisting  of  the  Ogishke  conglomerate,  the  Agawa 
formation,  and  the  Knife  Lake  slates)  and  the  Upper  Huronian  (Animikie) 
rocks.  These  rocks  come  closest  together  in  the  vicinity  of  Gobbemichi- 
gamma  Lake,  and  here,  if  at  all,  their  relations  Avere  to  be  determined. 
A  considerable  time  was  therefore  spent  in  the  study  of  the  rocks  in  this 
vicinity.  Unfortunately  where  the  rocks  of  the  series  come  closest  together 
the  Lower  Huronian  is  represented  by  conglomerates  which  give  no  good 
strikes  and  dips,  and  the  Animikie  is  represented  by  the  metamorphosed 
iron  formation  which  lies  at  its  bilse  in  this  district. 

In  the  area  referred  to  the  Lower  Huronian  rocks  are  extremely  folded, 
and  where  this  series  is  in  contact  with  the  Animikie,  the  vertical  or  very 
steejoly  dipping  rocks  of  the  Lower  Huronian  were  found  to  strike  in  such 
a  direction  on  the  east  side  of  Gobbemichigamma  Lake  as  to  bring  them 
very  nearly  at  I'ight  angles  against  the  Animikie,  which  has  a  very  low  dip 
to  the  south,  with  a  strike  slightly  north  of  east.  Only  in  two  places 
were  the  Ogishke  conglomerate  of  the  Lower  Huronian  and  the  iron- 
bearing  formation  of  the  Upper  Huronian  series  observed  in  contact,  and 


THE  LOWER  HURONIAN.    .  307 

in  botli  places  there  is  a  ver}^  striking  difference  in  the  Hthologic  character 
of  the  two  rocks.  The  iron-bearing  foraiation  is  made  np  essentially  of 
beds  of  magnetite  alternating  with  very  quartzose  bands,  whereas  the 
conglomerate  is  the  normal  greenstone  conglomerate,  although  very  much 
altered.  There  is  no  transition  between  the  two,  and  the  relationships 
appear  to  be  those  of  two  unconformable  series  of  rocks.  The  evidence  of 
this  unconformity  is,  however,  not  absolutely  conclusive  in  the  Vermilion 
district.  The  opinion  of  the  majority  of  those  who  were  studying  these 
rocks  for  the  Survey  was  in  favor  of  this  unconformable  relationship,  and 
in  this  respect  was  in  thorough  agreement  with  the  conclusions  reached  and 
already  published  by  the  Minnesota  survey.  However,  in  view  of  the  fact 
that  some  of  the  rocks  were  intensely  plicated,  it  was  recognized  that  it  was 
possible  for  them  to  change  both  their  strike  and  dip  within  a  very  short 
distance,  even  within  the  distance  which  separated  them  from  the  Animikie 
and  in  which  there  wei;e  no  exposures.  Such  a  change  might  possibly 
bring  them  into  perfect  conformity  with  the  Animikie.  In  view  of  this 
possibility  we  could  not  all  agi-ee  to  accept  the  unconformable  relationship 
as  proved.  In  1900,  shortly  after  work  was  begun  in  the  Mesabi,  Mr.  C.  K. 
Leith  had  the  good  fortune  to  find  a  place  where  the  relationship  between 
these  two  series  is  unmistakably  shown."  At  this  point,  north  of  Biwabik, 
the  vertical  Lower  Huronian  beds  were  found  overlain  by  the  low  south- 
ward-dipping rocks  of  the  Upper  Huronian  series,  with  a  thin  basal 
conglomerate  at  the  bottom  The  correctness  of  the  opinions  pre^aously 
held  were  thus  demonstrated  beyond  all  doubt. 

RELATIONS  TO   THE   KEWEENAW  AN. 

Belations  to  the  Keweenawan  gahhro. — In  Ts.  63  and  64  N  ,  Rs.  8  and  9 
W.,  the  Ogishke  conglomerates  and  Knife  Lake  slates  are  found  in  manv 
places  almost  in  juxtaposition  with  the  Duluth  gabbro  mass  of  Minnesota. 
In  no  cases  were  actual  contacts  observed  between  them,  as  invariably  a 
topographic  depression,  occupied  either  jnerely  by  lower  ground  or,  as  in 
most  cases,  by  water,  intervened.  The  Keweenawan  gabbro  has  been  long- 
recognized  as  one  of  the  youngest  rocks  occumng  in  Minnesota.  Where 
the  conglomerates  and  gabbro  are  in  contact  the  gabbro  has  been  found  to 
metamorphose  the  conglomerates  and  slates  very  extensively,  and  hence 

«Mon.  U.  S.  Geol.  Survey  Vol.  XLIII,  1903,  p.  181. 


308  THE  VERMILION  IKON-BEARING  DISTRICT. 

the  conclusion  is  unavoidable  that  the  gabbro  is  very  much  younger  than 
the  Ogishke  conglomerate  and  Knife  Lake  slates. 

Relations  to  basic  dikes. — Cutting  through  the  Ogishke  conglomerate  and 
Knife  Lake  slates  at  various  places,  basic  dikes  of  the  character  of  dolerites 
have  been  observed.  These  dikes  are  of  exactly  the  same  nature  as  those 
which  are  found  cutting  thi-ough  the  gabbro  which  represents  the  youngest 
member  of  the  series  in  the  Vermilion  district,  excluding,  of  course,  as  was 
done  in  the  statement  at  the  beginning  of  this  monograph,  the  glacial 
deposits.  Since  the  dikes  in  the  conglomerate  are  lithologically  the  same 
as  those  cutting  the  gabbro,  they  are  assumed  to  be  of  the  same  age, 
although  direct  relationship  between  the  dikes  in  the  conglomerate  and  slates 
and  those  in  the  gabbro  have  never  been  observed. 

AGE  OF  THE  LOWER  HURONIAN   SEDIMENTS. 

The  descriptions  given  in  the  above  paragraphs  of  the  relations 
existing  between  the  conglomerates  and  slates  of  the  Lower  Huronian  and 
the  adjacent  formations  throughout  the  district  enable  us  to  make  with 
confidence  the  following  summary  statement  concerning  the  relative  strati- 
gi'aphic  position  of  these  sediments  in  the  Vermilion  district.  Since  they 
lie  unconformably  above  the  rocks  of  Archean  age,  they  must  of  necessity 
be  younger  than  the  Archean  rocks.  They  are  older  than  the  Snowbank, 
Cacaquabic,  and  Giants  Range  granites,  which  cut  and  metamorphose 
them,  and  older  than  some  basic  dikes  which  are  intrusive  in  them.  The 
chief  interest,  however,  is  in  their  relationship  to  the  Animikie  sediments, 
which  are  very  generally  recognized  as  being  of  Upper  Huronian  age. 
The  sediments  here  classed  as  Lower  Huronian  are  unmistakably  of  an 
older  period  of  formation  than  these  Animikie  sediments,  since  these 
Animikie  sediments  rest  unconformably  upon  them,  as  is  shown  by  the 
relations  observed  in  the  adjacent  Mesabi  district.  Hence  the  tlii-ee 
conformable  formations,  the  Ogislike  conglomerate,  the  Agawa  formation, 
and  the  Knife  Lake  slates  form  one  series  of  rocks  of  Lower  Hui'onian  age. 
In  the  eastern  part  of  the  Vermilion  district  these  sediments  bear  the  same 
relations  to  the  adjacent  formations  as  in  the  western  part.  They  lie  at 
the  base  of  the  Algonkian  sediments,  and  rest  miconforniabl)-  upon  the 
Archean  rocks,  and  are  correlated  with  the  Lower  Huronian  series  of  the 
rest  of  the  Lake  Superior  region. 


THE  LOWER  HURONIAN.  309 


OGISHKE  CONGLOMERATE. 
PETROGRAPHIC    CHARACTERS. 


Macroscopic  characters. — The  Ogishke  cong-lomerate  varies  from  a 
coarse  bowlder  conglomerate  with  bowlders  up  to  20  iuches  iu  diameter,  as 
shown  on  the  southwest  side  of  Cache  Bay  of  Saganaga  Lake,  down 
through  all  intermediate  gradations  of  coarseness  into  rocks  which  are 
designated  as  grits,  and  thi-ough  these  into  slates.  The  grits  are,  of  course, 
interbedded  with  the  conglomerates,  but  no  attempt  has  been  made  to 
separate  them  from  the  conglomerates  on  the  majjs  where  it  was  recognized 
that  they  occurred  in  very  subordinate  quantity.  The  conglomerates  contain 
a  great  variety  of  pebbles.  We  find  among  these  a  great  number  of  kinds 
of  altered  basic  eruptives,  both  massive  and  schistose,  coarse  and  fine 
grained,  porphyritic  and  nonporjDhyritic,  amygdaloidal  and  nonamygdaloi- 
dal,  some  showing  flowage  lines  produced  by  parallelism  of  the  feldspars, 
and  others  with  sjjherulitic  structure.  Among  the  most  striking  of  these 
are  the  porphyritic  rocks  in  which  the  feldspar  and  hornblende  are  the 
])henocrysts  and  occur  either  alone  or  together.  Upon  one  ledge  seven 
different  kinds  of  greenstones  were  counted.  The  granites  which  occur  in 
pebbles  and  bowlders  in  the  conglomerates  show  varieties  ranging-  from 
coarse  and  fine  evenly  grained  to  porphyritic  and  nonporphyritic  forms. 
There  are  several  kinds  of  fine-grained  acid  porphyries  also.  A  few 
slate  fragments  and  two  fragments  of  a  conglomerate  were  likewise  seen 
in  the  coarse  elastics.  Black  and  gray  chert,  jasper,  vein  quartz,  and  a 
number  of  fine-grained  gray  pebbles,  whose  characters  were  undetermined, 
occur  associated  with  those  mentioned. 

The  brilliant  red-jasper  fragments  lying  in  the  green  matrix  give  the 
conglomerate  a  very  handsome  appearance.  With  this  jasper-bearing 
conglomerate,  and  a  phase  grading  over  into  the  Knife  Lake  slates,  there 
occiirs  a  dark-green  medium-grained  graywacke  with  a  faint  speckling,  due 
to  the  small,  bright-red  fragments  of  jasper  scattered  through  it.  The 
amount  of  small  jasper  fragments  varies  in  quantity,  being  rare  in  some 
cases,  and  in  others  so  numerous  as  to  influence  very  markedly  the  color 
of  the  graywacke. 

Many  of  the  jasper  fragments  which  occtir  in  this  conglomerate  possess 
a  well-developed  zonal  structure.     The  centers  of  the  fragments  are  red 


310  THE  VERMILION  IRON-BEARING  DISTRICT. 

and  the  peripheries  are  bhxck,  producing  a  xerj  striking  appearance.  This 
zonal  structure,  moreover,  is  parallel  to  the  irregular  contours  of  the 
pebbles.  The  alteration  is  evidently  due  to  the  reduction  of  the  iron  oxide 
(Fe203zrhematite)  to  magnetite  (FegOa  FeO).  The  zonal  structure  in  these 
fragments  is  verj^  good,  and  must  have  been  formed  after  the  jasper 
had  acquired  its  present  fragmentary^  character,  as  the  zones  run  parallel 
with  the  irregular  margins  of  these  fragments.  Some  of  the  fragments  are  a 
foot  long,  although  the  majority  of  them  are  only  a  few  inches  in  diameter. 
Where  there  is  brilliant  red  jasper  lying  near  the  conglomerates  it  is 
natural  to  look  for  some  iron-bearing  formation  as  the  local  source  of  the 
23ebbles.  However,  a  great  deal  of  the  conglomerate  occurring  upon  and 
in  the  vicinity  of  Ogishke  Muncie  Lake  has  as  its  most  striking  constituent 
brilliant-red  jasper  pebbles,  and  yet  there  is  no  known  typical  iron-bearing 
formation  nearer  than  that  which  occurs  on  Otter  Track  Lake,  5  miles 
away  to  the  northwest.  The  intervening  area  is  underlain  by  finer  sedi- 
ments— graywackes  and  slates  for  the  most  part.  The  brilliant  jasper  from 
this  vicinity  is  in  its  general  character  similar  to  that  of  Lower  Huronian 
age  which  occurs  at  Soudan,  Ely,  and  other  places  in  the  district.  It  seems 
scarcely  reasonable  to  derive  these  pebbles  from  a  source  so  far  distant  as 
the  exposure  on  Otter  Track  Lake,  especially  as  the  jasper  occurs  in  such 
large  quantity  in  the  sediments  exposed  in  the  area  extending  ajjproxi- 
mately  from  West  Gull  Lake  southwestward  to  Cacaquabic  Lake.  Win- 
chell"  reports  the  occurrence  of  a  small  quantity  of  jasper  upon  Townline 
Lake,  but  search  failed  to  reveal  it.  IVloreover,  the  Archean  area  to  the 
south  and  east  of  these  sediments  has  been  hunted  over  for  the  Soudan 
formation,  which  it  was  supposed  might  be  b'ing  in  troughs  within  it,  but 
it  was  not  found.  The  Soudan  formation,  if  it  ever  existed  in  this  area, 
and  it  is  highly  probable  that  it  did  exist,  has  been  deeply  buried  under 
the  sediments  or  completely  removed  liy  erosion.  The  probability  of 
such  a  removal  will  be  seen  to  be  great  when  we  consider  the  enormous 
thickness  of  sediments  whicli  lie  west  and  northwest  of  the  Archean  and 
which  consequently  indicate  a  long  period  of  erosion  and  deposition. 

The  matrix  of  the  Ogishke  conglomerate  is  the  finely  triturated 
material  derived  from  the  various  rocks  whicli  have  been  mentioned  as 
occurring  in  pebbles  in  the  conglomerate. 


"Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Sixteenth  Ann.  Rept.,  1S85,  p.  31.5. 


THE  LOWER  HURONIAN.  311 

The  proportion  of  the  different  kinds  of  pebbles  varies  greatly,  and 
consequently  we  find  conglomerates  of  very  different  chemical  and  physical 
aspects.  The  dependence  of  the  character  of  the  conglomerate  on  the 
petrographic  nature  of  the  adjacent  rocks  from  which  it  has  been  derived  is 
well  illustrated  as  we  follow  northward  the  line  of  contact  between  the 
granite  of  Saganaga  Lake  and  Ogishke  conglomerate  on  Cache  Bay  of 
Saganaga  Lake,  where  we  get  the  Ogishke  conglomerate  in  contact  with 
the  Ely  greenstone,  which  has  been  cut  by  numerous  dikes  of  the  granite 
of  Saganaga  Lake.  As  we  go  in  this  direction  we  find  that  the  basal 
conglomerate  contains  occasionally  fragments  of  g'reeustone,  and  these 
become  more  and  more  numerous,  the  granite  appearing  in  proportionally 
smaller  quantity  as  the  area  in  which  the  greenstone  occurs  is  approached. 
Finally,  when  we  get  well  within  the  area  in  which  the  greenstone  occurs, 
the  conglomerate  is  made  up  cliiefly  of  greenstone  fragments,  the  matrix 
likewise  beino-  detrital  material  from  the  srreenstone  with  onlv  an  occasional 
granite  pebble,  derived  probably  from  the  gi-anite  dikes  which  traverse 
the  greenstone  or  possibly  transported  from  the  main  granite  area.  The 
changeable  cliaracter  of  the  basal  conglomerate  within  a  very  limited  area, 
its  character  depending  on  the  petrographic  nature  of  the  surrounding 
rocks,  is  also  seen  here,  as  above  described,  and  shows  the  uncertainty  of 
correlations  which  depend  on  the  similarity  of  the  lithologic  character  of 
sedimentary  deposits  occurring  in  widely  separated  areas.  The  differ- 
ences in  the  Ogislike  conglomerate  at  various  localities  is  clearly  due  to 
the  varying  character  of  the  immediately  adjacent  rocks  from  which  the 
conglomerates  have  been  derived.  This  is  shown  at  a  number  of  places. 
Thus  in  some  conglomerates  all  of  the  pebbles  are  greenstone,  and  the 
matrix  is  likewise  made  up  of  the  dust  from  these  greenstones,  so  that  the 
resulting  rock  is  green,  with  pebbles  showing  various  textures,  such  as  occur 
in  basic  rocks.  Jasper  occurs  in  numerous  fragments  in  these  rocks,  and 
their  brilliant  color  offers  a  very  striking-  contrast  to  the  usual  monotonous 
green  of  the  conglomerates.  Here  and  there  a  pebble  or  bowlder  of  granite 
will  appear,  and  again  there  may  be  an  approximately  even  mixture  of 
pebbles  of  granite  and  greenstone.  In  other  places  the  frag-ments  of  granite 
are  present  in  such  quantity  that  they  are  the  predominant  bowlders.  In 
such  places  the  matrix  likewise  is  found  to  have  changed  from  the  green  of 
the  greenstone  conglomerates  to  the  gray  or  pinkish  color  characteristic  of 


312  THE  VERMILION  IRON-BEARING  DISTRICT. 

gi-anite  debris.  The  most  noticeable  differeuces  in  the  appearance  of  the 
Ogishke  cong'lomerate  are  between  those  varieties  which  are  made  up  almost 
exclusively  of  greenstone,  which  are  therefore  g-reeu  in  cokir,  and  those  in 
which  the  granite  bowlders  form  the  g-reater  portion,  such  varieties  having  a 
gray  to  reddish  color  on  weathered  sxirfaces.  Between  these  there  are,  of 
course,  all  gradations.  In  general  the  gray,  bluish-  to  greenish-gray,  slate- 
colored,  and  green-colored  rocks  predominate.  These  different  kinds  of 
conglomerates  were  noted  before  they  were  all  correlated,  and,  for 
convenience,  the  conglomerate  made  up  essentially  of  greenstone  pebbles, 
which  is  so  typically  developed  in  the  vicinity  of  Moose  Lake,  was  spoken 
of  as  the  greenstone  conglomerate.  These  greenstone  conglomerates  were 
for  a  while  rather  puzzling,  as  it  was  not  easy  to  determine  whether  they 
were  volcanic  tuffs  or  true  sedimentary  rocks,  in  the  one  case  contempo- 
raneous with  and  in  the  other  older  than  the  greenstones  with  which  they 
were  associated.  Grant,  after  having  studied  the  district,  reached  the 
conclusion  that  the  clastic  rocks  on  the  south  flank  of  the  Archeau  tongue 
lying  south  of  Gull  Lake  and  extending  thence  eastward  were  tuffs  derived 
from  these  greenstones."  Field  work  having  for  its  object  the  determination 
of  this  particular  point  has  been  carried  on,  and  as  a  result  sufficient 
indications  of  the  sedimentary  origin  of  the  elastics  have  been  found  to 
justify  their  classification  as  conglomerates.  Moreover,  in  numerous  other 
places  elsewhere  in  the  district,  interbedding  of  the  finely  bedded  sediments 
with  these  conglomerates  and  gi-adations  between  them  have  been  observed, 
so  that  there  can  be  absolutely  no  doubt  that  they  are  normal  sediments. 
The  conglomerate  at  Ogishke  Muncie  Lake  contains  pebbles  of  greenstone, 
granite,  jasper,  and  otlier  varieties  of  rocks,  find  is  the  normal  basal  conglom- 
ei'ate  of  the  Lower  Huronian  for  the  eastern  part  of  the  Vermilion  district. 
Microscopic  characters. — A  certain  amount  of  microscopic  study  was 
made  of  the  conglomerates.  This  consisted  primarily  in  the  determination 
of  the  characters  of  the  pebbles  and  of  the  matrix.  Li  addition  to  the 
constitiients  recognizable  macroscopically,  which  have  already  been  enum- 
erated, the  microscope  discloses  the  presence  of  fragments  of  basaltic  lavas 
with  various  microscopically  recognizable  textures,  spherulitic  rhyolite, 
rhyolite-porphyry,  pieces  of  quartz  and  feldspar  in  pegmatitic  intergrowth 


"Geology  of  the  eastern  end  of  the  Mesabi  iron  range  in  Minnescita,  by  U.  8.  Grant:  Engineers' 
Yearbfwk,  University  of  Minnesota,  1898,  p.  54. 


THE  LOWER  HURONIAN.  313 

(presumably  derived  from  pegmatitic  granites),  round  to  subangular  grains 
of  cloudy  feldspar,  quartz,  ragged  pieces  of  biotite  altering  to  chlorite, 
green  hornblende,  augite,  apatite,  zircon,  and  a  large  amount  of  fine  inde- 
terminable interstitial  material  derived  from  the  trituration  of  the  various 
materials  already  mentioned.  From  the  decomposition  of  this  interstitial 
material,  as  well  as  from  the  associated  larger  fragments,  there  has  been 
produced  the  secondary  minerals  chlorite,  ealcite,  actinolite,  sericite, 
epidote,  quartz,  feldspar,  and  pyrite,  which  occur  in  large  quantity  in  the 
sediments.  The  prevailing  green  tones  of  the  sediments  is  especially  due 
to  the  very  large  quantity  of  the  green  hornblende,  epidote,  chlorite, 
augite,  and  sericite  which  is  present.  In  a  few  cases  the  well-bedded  gray- 
wackes  associated  with  the  conglomerates,  and  especially  those  near  the 
conglomerates  that  are  composed  chiefly  of  greenstone  fragments,  are  found 
to  consist  very  largely  of  fragments  of  crystals  of  hornblende,  augite,  and 
feldspar,  with  an  occasionally  well-preserved  entire  crj^stal.  Such  sedi- 
ments resemble  the  crystalline  tuffs  of  volcanic  origin.  The  beautiful 
bedding  and  association  with  other  sediments  show  clearly  that  these  rocks 
are  water-deposited  sediments. 

METAMOEPHISM    OF    THE    OGISHKE    CONGLOMERATE. 

It  should  be  borne  in  mind  that  the  conglomerates  thus  far  descnbed 
are  by  no  means  in  their  original  condition,  but  have  been  extensively 
metamorphosed.  This  metamorphism  has  been  that  produced  chiefly  by 
cementation,  due  largely  to  infiltration  of  silica  and  some  calcium  carbonate 
and  chemical  change  of  the  constituents  producing  new  minerals,  and  also 
secondary  enlargements  of  the  old  minerals.  As  the  result  of  these  changes 
and  additions  the  conglomerates  have  been  thoroughly  indurated.  This  is 
the  widespread  metamorphism  which  is  common  throughout  all  of  these 
Lower  Huronian  sediments.  The  kinds  of  metamorphism  now  to  be 
described  are  exceptional;  they  are  not  those  most  common  in  these  con- 
glomerates: 

The  Ogishke  conglomerate  has  been  metamorphosed  both  as  the  result 
of  orogenic  movements  and  in  consequence  of  the  intrusion  through  it  of 
various  granites,  as  well  as  of  its  contact  with  the  Duluth  gabbro  of  Kewee- 
nawan  age.  The  result  of  crustal  movements  is  well  shown  in  the  schis- 
tose conglomerate  which  may  be  seen  at  the  easternmost  jDoint  south  of  the 


314  THE  VERMILION  IRON-BEARING  DISTRICT. 

channel  between  Birch  Lake  and  Sucker  Lake.  Here  the  conglomerate 
consists  almost  exclusively  of  fragments  of  the  greenstone  which  lies  on  the 
north  side  of  the  channel.  An  occasional  fragment  of  vein  quartz  is  present. 
This  rock  has  been  so  extremely  mashed  that  the  pebbles  have  been  rolled 
out  and  flattened,  and  it  is  now  fairly  difficult  to  recognize  its  true  character. 
Its  fragmental  character  is  best  shown  on  the  poi'tions  of  the  exposure 
where  one  gets  sections  transverse  to  the  direction  of  greatest  flattening  in 
the  pebbles.  The  difficulty  here  is  increased  by  the  fact  that,  as  stated 
above,  the  conglomerate  consists  almost  exclusively  of  the  greenstone,  the 
matrix,  of  course,  being  derived  from  the  same  source  and  consisting  of 
the  same  material. 

A  mashed  cong'lomerate  similar  to  this  in  every  respect  was  seen  at  a 
number  of  places  on  the  very  irregular  stream  that  connects  Cache  Bay  of 
Saganaga  Lake  with  Saganagons  Lake  in  Canada.  The  best  place  at  which 
to  see  this  conglomerate  is  on  the  south  shore  of  the  stream  where  it  turns 
northeastward  and  flows  into  the  southwest  bay  of  Saganagons  Lake.  In 
this  vicinity  the  greenstone  and  the  overlying  sediments  have  been  very 
closely  infolded,  and  as  a  result  of  the  folding  excessive  shearing  has  taken 
place  along  the  limbs  of  the  folds.  Here,  again,  if  the  conglomerate  is 
viewed  transversely  to  the  bedding,  its  nature  can  be  readily  recognized. 
On  some  of  the  clifi"s,  liowever,  which  run  parallel  with  the  bedding  of  the 
conglomerate,  and  essentially  with  that  of  the  schistosity,  the  conglomeratic 
nature  is  not  so  readily  recognized.  The  pebbles  occasionally  stand  out  as 
more  or  less  rounded  patches  on  the  cliff"  face,  but  very  connnonly  blend 
with  the  matrix  so  nicely  that  the  rock  appears  almost  homogeneous. 

Naturally  where  the  conglomerate  is  made  up  of  a  great  variety  of 
pebbles,  its  true  character  may  be  recognized  with  greater  ease  than  in  the 
above-mentioned  instances.  Usually  where  the  conglomerate  has  been 
folded,  the  pebbles  have  not  been  much  affected.  The  conglomerate  may 
in  general  have  a  schistose  character,  and  tliis  schistosity  usualh^  agrees, 
approximately,  with  the  long  direction  of  the  pebbles.  Closely  exannned, 
it  will  be  found  that  the  pebbles  themselves  are  in  most  instances  not  even 
fractured,  but  preserve  their  original  shape  perfectly.  Pebbles  of  granite 
have  been  obtained  from  this  conglomerate  which  were  as  symmetrical  in 
shape  and  apparently,  to  the  naked  eye,  as  fresh  in  character  as  pebbles 
obtained  from  a  modern  shingle  beach  of  Lake  Superior.     The  pressure 


THE  LOWER  HURONIAN.  315 

on  the  conglomerate  has  been  reheved  by  movement  in  the  matrix,  and 
this  matrix  has  in  most  cases  been  rendered  perfectly  schistose.  It  will  be 
found  that  the  schistosity,  when  it  approaches  a  pebble,  gradually  bends 
so  as  to  run  around  the  ends  of  the  pebble,  and  then  upon  the  flat  sides 
continues  in  its  normal  direction. 

The  contact  metamorphism  resulting  from  the  intrusion  of  io-neous 
rocks  seems  to  have  been  more  far  reaching  in  its  character — at  least  so  far 
as  it  has  produced  changes  in  the  petrographic  character  of  the  cono-lom- 
erate — than  the  metamorphi,sm  due  simply  to  orogenic  movement.  In  all 
probability  the  action  of  the  intrusives  is  complicated  by  the  fact  that  their 
intrusion  took  place  subsequent  to  some  orogenic  movements,  so  that  thev 
acted  on  rocks  which  had  already  been  somewhat  metamorphosed.  The 
best  area  in  which  to  study  this  contact  action  is  in  the  vicinity  of  Snow- 
bank Lake.  Snowbank  Lake  lies  in  a  granite  massive,  which  has  received 
its  name,  the  Snowbank  granite,  from  the  lake.  The  Ogishke  conglom- 
erate surrounds  this  lake  and  is  exposed  with  bare  surfaces  over  large 
areas.  At  a  considerable  distance  away  from  the  lake  the  conglomerate 
possesses  its  normal  characters,  but  as  the  lake  is  approached  it  will  be 
seen  that  gradually  it  changes.  Tliis  change  is  for  the  most  part  a  petro- 
gi'aphic  one,  and  has  consisted  of  the  production  of  micaceous  and  horn- 
blendic  schists  from  the  finer-grained  sedim.^nts  associated  with  the 
conglomerates  and  from  the  fine  matrix  between  the  pebbles  of  the  con- 
glomerates. The  pebbles  in  the  conglomerates  have  also  been  altered, 
certain  kinds,  of  course,  very  much  more  than  others.  In  general  these 
finer  materials  are  now  well-developed  mica-schists  in  which  the  cono-lom- 
eratic  character  can,  however,  be  readily  recognized  by  the  presence  of 
comparatively  unaltered  granite  pebbles.  This  alteration  has  reached  its 
extreme  where  the  conglomerates  are  nearest  to  the  granite. 

Long  after  the  intrusion  of  the  granite  and  the  metamorphism  of  the 
conglomerates,  the  Keweenawan  gabbro  was  intruded,  and  it  has  in  turn 
modified  the  conglomerates,  which  were  already  metamorphosed  by  orogenic 
movements  and  by  the  granite.  The  effect  of  the  contact  action  of  the  gab- 
bro has  extended  for  a  considerable  distance  from  the  present  exposm-es  of 
the  gabbro.  The  exact  distance  can  not  be  determined  with  certainty,  as 
its  metamorphism  blends  with  that  produced  by  the  granite.  Possibly 
a  more  detailed  field  and  petrographic  study  than  was  warranted  in  the 


316  THE  VERMILION  IRON-BEARING  DISTRICT. 

jjreseut  case  might  enable  a  boundary  line  to  be  drawn  between  the  con- 
glomerates metamorphosed  by  the  granite  only  and  those  metamorphosed 
by  both  the  granite  and  gabbro.  The  effect  of  the  gabbro  seems  to 
have  extended  at  least  as  far  north  as  the  northwest  shore  of  Disappoint- 
ment Lake.  Reference  has  already  been  made  to  the  beautiful  exposures 
of  conglomerate  at  this  place.  The  matrix  of  this  conglomerate  contains 
biotite  and  hornblende  very  abundantly,  and  the  pebbles  and  bowlders  of 
the  conglomerate  are  to  a  great  extent  of  a  hornblendic  rock  in  which  in 
many  places  large  porphyritic  hornblendes  have  been  produced.  On  the 
weathered  surface  the  pebbles  generally  decay  faster  than  the  matrix,  and 
hence  are  removed,  leaving  numerous  roundish  depressions.  The  rock  has 
not  been  very  strongly  mashed,  for  while  the  longer  dimensions  of  the 
pebbles  are  in  the  planes  of  schistosity,  one  could  not  say  that  the  lesser 
dimension  in  the  other  direction  is  not  explicable  as  due  to  the  original 
shingling  action  of  the  pebbles.  The  rock  is  very  irregularly  veined  by 
quartz,  and  here  and  there  by  some  granitic  veins  derived  from  the  Snow- 
bank granite.  All  these  factors  give  the  rock  a  very  rough,  knotty 
appearance  on  the  weathered  surface.  While  in  general  the  outlines  of  the 
pebbles  can  be  readily  traced,  nevertheless,  when  they  are  broken  the 
fractures  extend  cleanly  through  the  pebbles  and  matrix  alike.  This  shows 
the  close , union  between  the  matrix  and  the  pebbles  which  has  taken  place 
as  the  result  of  metamorphism.  This  is  further  shown  by  the  fact  that  m 
some  cases  secondary  porphyritic  hornblendes,  which  are  produced  in 
certain  of  the  pebbles  and  in  the  matrix  alike,  will  be  found  to  extend  from 
the  pebble  across  the  contact  into  the  matrix.  As  we  go  southward — in 
other  words,  as  we  get  closer  to  the  gabbro — a  study  of  the  rocks  on  the 
small  i.slands  in  Disappointment  Lake  and  on  the  south  shore  shows  that 
the  clastic  nature  of  the  rocks  is  not  so  apjDarent  here.  A  microscopic 
study  of  these  rocks  shows  that  there  have  been  produced  in  them  in  large 
quantity  minerals — hypersthene,  green  and  brown  hornblende,  brown  mica, 
augite,  magnetite — whose  origin  is  clearly  due  to  the  action  of  the  gabbi-o. 
Similar  rocks  may  be  studied  in  the  area  extending  from  the  north  shore 
of  the  Kawishiwi  River  in  sees.  16,  17,  and  20,  T.  63  N.,  R.  9  W., 
near  the  shore.  They  are  extremely  metamorphosed  and  it  is  only  by 
rather  close  observation  that  one  can  recognize  tlieir  conglomeratic  nature. 
The  pebbles  and  the  matrix  of  the  rocks  consist  to  a  great  extent  of  the 


THE  LOWER  HURONIAN.  317 

same  material,  and  as  the  result  of  the  metamorphism  essentially  the  same 
new  minerals  have  been  produced  in  them,  and  this  production  of  new 
minerals  has  tended  to  render  the  characters  of  the  rock  more  uniform.  It 
is  only  here  and  there,  where  a  pebble  occurs,  whose  mineral  composition 
was  not  so  extremely  changred  by  the  metamorphism  that  the  conglomeratic 
chai'acter  of  the  I'ock  can  be  distinctly  recognized.  In  such  cases  these 
pebbles  seem  to  withstand .  the  weathering  better  than  does  the  adjacent 
material  in  which  have  been  produced,  as  above  stated,  the  basic  minerals 
associated  with  the  gabbro.  This  portion  of  the  rock  weathers  more  readily 
than  the  less  affected  pebbles  which  stand  out  from  the  rest  of  the'  rock 
and  show  their  true  pebble  characters. 

THICKNESS    OF   OGISHKE    COXGLOJIEEATE. 

No  data  have  been  obtained  that  would  enable  us  to  make  an  accurate 
determination  of  the  thickness  of  the  conglomerate.  In  places  it  is  wanting 
or  is  represented  by  a  few  feet  of  rock  at  most,  and  from  this  it  runs  up  to 
a  thickness  of  possibly  a  thousand  or  more  feet. 

INTERESTING   LOCALITIES. 

The  portage  between  Moose  and  Flask  lakes  is  a  good  place  at  which 
to  study  the  Ogishke  conglomerate  as  developed  south  of  Moose  Lake.  A 
short  distance  east  of  the  portage  landing  on  Moose  Lake  as  the  hill  is 
ascended  we  find  slates  similar  to  those  occurring  along  the  east  shore, 
which  appear  to  grade  into  a  fine  conglomerate  and  then  into  a  coarse 
conglomerate  which  crowns  the  brow  of  the  hill.  The  rocks  along  tliis 
gradation  zone  are  much  mashed,  so  that  one  can  not  be  certain  that  the 
change  is  due  to  actual  continuous  sediments  difi^ering  only  in  coarseness. 
The  conglomerate  is  very  coarse  at  this  place,  having  bowlders  u]j  to  2^ 
feet  in  diameter.  The  fragments  in  the  conglomerate  are  of  many  different 
rocks — various  kinds  of  porphyries,  granites,  many  diflPerent  varieties  of 
greenstone,  jasper,  an  occasional  slate  fragment,  and  two  fragments  of 
a  conglomeratic  or  brecciated  rock,  the  fragments  and  matrix  in  these  two 
pieces  being  very  much  alike,  and  apparently  both  derived  from  green- 
stone. Still  farther  along  the  trail  the  same  conglomerate  occurs  at 
numerous  places,  and  here  and  there  are  exposures  of  much  contorted 
slates  associated  with  the  conglomerates.  Some  dikes  of  granite- porphyry 
with  large  quartz   phenocrysts  may   also  be  observed  cutting  the  older 


318  THE  VERMILION  IRON-BEARING  DISTRICT. 

sediments.  After  leaving  Flask  Lake  one  finds,  in  traversing  the  portage 
between  Flask  Lake  and  Snowbank  Lake,  a  number  of  good  exposures 
showing  a  i-ather  coarse  greenstone  conglomerate,  which  is  well  developed 
in  this  vicinity.  These  conglomerates  very  rarely  contain  any  pebbles  of 
granite  or  jasper,  most  of  the  fragments  being  of  greenstone,  and  in  this 
respect  the  conglomerates  are  different  from  those  occurring  farther  north 
and  west,  nearer  Moose  Lake.  They  have  also  suffered  mqre  from  meta- 
morphism,  since  they  are  nearer  to  the  main  Snowbank  granite  massive 
which  has  intruded  them. 

The  northwest  shores  of  Disappointment  Lake  afford  the  best  places 
in  the  Vermilion  district  for  studying  the  Ogishke  conglomerate.  At  the 
time  of  the  survey  here  reported  the  country  had  been  recently  bui'ned 
over  and  the  hills  were  practically  bare.  There  was  a  little  scanty 
vegetation,  but  one  could  see  bare  rock  exposed  in  great  flat  or  slightly 
rounded  surfaces  nearly  everywhere.  Great  beds  of  coarse  bowlder 
conglomerates  were  exposed,  grading  into  finer-grained  conglomerates, 
and  these  through  gray wackes  into  slates,  to  be  succeeded  by  a  repetition 
of  these  gradations.  The  strike  of  the  bedding  is  very  uniformly  N.  20°  W., 
the  dip  varies  from  75°  west  to  80°  east,  but  is  very  commonly  nearly 
vertical.  The  conglomerate  at  Disappointment  Lake  differs  from  the  typical 
Ogishke  in  respect  to  the  absence  from  it  of  the  jasper  which  is  so  common 
in  the  typical  Ogishke.  No  pebbles  of  this  kind  were  found  on  Disappoint- 
ment Lake.  The  pebbles  consisted  chiefly  of  varieties  of  granite  and 
porphyry,  and  especially  of  numerous  varieties  of  greenstone.  In  fact, 
many  of  the  beds  of  conglomerate  consisted  exclusively  of  pebbles  of 
different  varieties  of  greenstone,  and  the  matrix  between  the  pebbles 
consisted  of  finer  detrital  matter  derived  from  the  same  source.  The 
conglomerates  had  been  intruded  b}'  a  number  of  dikes,  basic  as  Avell 
as  acid,  and  they  had  been  metamorphosed  by  the  Snowbank  granite, 
from  which  the  granite  dikes  are  offshoots,  and  subsequent  to  this 
metamorphism  had  been  further  metamorphosed  b}'  the  great  Kewee- 
nawan  gabbro.  Consequently  the  nearer  one  approaches  the  Snowbank 
granite  the  greater  the  metamorphism,  and  the  changed  character  of  the 
sediments  is  still  further  increased  as  one  goes  along  the  margin  of  the 
granite  mass  and  approaches  nearer  to  the  gabbro.  In  some  places,  even  at 
considerable  distances  from  both  of  these  intrusive  rocks,  the  conglomerates 


THE  LOWER  HURONIAN.  319 

haTO  been  so  much  altered  that  their  true  characters  could  be  detected 
onh"  on  careful  examination.  The  greater  the  variety  of  pebbles  the  more 
difficult  it  becomes  to  conceal  the  true  character  of  the  conglomerate,  as 
even  in  the  most  exti'eme  cases  it  is  probable  that  some  one  or  more  of  the 
kinds  of  pebbles  may  retain  very  nearly  their  normal  characters.  But 
when,  as  was  frequently  found  to  be  true,  th^e  conglomerate  beds  are  made 
up  essentially  of  one  kind  of  rock,  the  greenstone,  and  when  this  has  been 
metamorphosed,  it  is  found  that  it  is  sometimes  difficult  to  recognize  the 
original  chai-acter.  In  fact,  in  such  a  case  as  this  large  secondary 
hornblende  crystals  were  found  to  have  been  produced  throughout  the 
conglomerate,  and  in  the  matrix  as  well  as  in  the  pebbles,  and  likewise 
grew  from  the  pebbles  out  into  the  matrix. 

From  Disappointment  Lake  north  to  the  vicinity  of  Ensign  Lake, 
and  from  Ensign  Lake  east  to  Lake  Cacaquabic,  there  are  a  number  of 
areas  outlined  on  the  map  (PL  II),  in  which  the  Ogishke  conglomerate 
is  exposed.  In  all  of  these  areas  the  conglomerate  consists  essentially 
of  greenstone  pebbles,  with  gTanite  pebbles  secondary  in  abundance. 
Jasper  is  practically  wanting.  In  general  appearance  the  conglom- 
erate is  green  as  the  result  of  the  preponderance  of  the  greenstone,  and 
since  the  brilliant-red  jasper  pebbles  are  wanting  it  does  not  present  the 
appearance  of  the  typical  Ogishke  conglomerate.  On  the  north  shore  of 
Cacaquabic  Lake  the  Ogishke  conglomerate  is  also  exjjosed.  Here  the  green- 
stone is  practically  the  only  kind  of  pebble  iu  the  rock,  and  the  conglomerate 
is  very  similar  to  some  of  the  greenstone  tufFs  of  other  districts  of  Lake 
Superior.  It  was  while  studying  this  rock  that  Van  Hise"  observed  the 
secondary  enlargement  of  hornblende  fragments.  On  the  south  shore  of 
the  long  east  arm  of  Cacaquabic  Lake  the  normal  Ogishke  conglomerate  is 
exposed  here  and  there,  and  shows  its  typical  characters  especially  well  at 
the  foot  of  the  high  cliff  about  half  a  mile  east  of  the  main  body  of  the  lake. 
Here,  especially  on  the  bowlders  which  lie  just  a  few  inches  or  feet  below 
the  surface  of  the  water,  the  jasper  pebbles,  with  their  brilliant  red  color, 
stand  out  conspicuously.  The  granite  pebbles  increase  in  quantity  and, 
as  above  stated,  the  rock  is  the  typical  Ogishke  conglomerate. 

At  the  narrows  of  the  lake  at  the  center  of  sec.  28,  T.  65  N.,  R.  6  W., 
is"  a  phase  of  the  Ogishke   conglomerate  different  from  those  heretofore 

"Am.  Jour.  Sci.,  3d  series,  Vol.  XXX,  1885,  pp.  231-2.35. 


320  THE  VERMILION  IRON-BEARING  DISTRICT. 

described.  This  is  made  up  exclusively  of  both  pebbles  and  matrix  of 
granite  debris.  The  matrix  contains  many  subangular  individuals  of 
feldspar,  derived  in  all  probabilit}'  fi-om  a  porphyritic  granite  or  feldspar- 
porphyry.  In  some  places  the  fine  graywacke,  with  the  jDorphyritic 
feldspars  which  are  associated  with  the  conglomerates,  simulates  very 
much  a  feldspathic  porphjiy.  East  of  this  lake  are  here  and  there 
exposures  of  Ogishke  conglomerate,  usually  in  rather  thin  beds,  asso- 
ciated with  graywackes  and  slates.  About  a  quarter  of  a  mile  south  of 
the  small  lake  just  west  of  Ogishke  Muncie  Lake,  the  Ogishke  conglom- 
erate is  again  exposed  in  large  masses  with  jasper  pebbles  present  in  great 
abundance.  This  cong'lomerate  extends  eastward  to  the  lake  and  along-  its 
south  shore,  and  eventually  is  connected  with  the  great  mass  of  conglom- 
erate to  the  east  of  Ogishke  Muncie  Lake.  As  we  go  from  the  west  end 
of  the  lake  eastward,  the  jasper  pebbles  rapidly  diminish  in  quantity,  aiid 
eventually  disappear,  so  that  the  conglomerate  exposed  on  the  south  side  of 
Ogishke  Muncie  Lake  is  made  up  chiefl}-  of  pebbles  of  different  varieties 
of  greenstone,  with  an  occasional  feldspathic  porphyry  and  granite- 
porj^hyry  pebble.  The  typical  Ogishke  conglomerate  also  occurs  north  of 
the  west  end  of  Ogishke  Muncie  Lake  and  is  likewise  exposed  along  the 
greater  portion  of  the  north  shore  of  the  lake.  Here  it  is  in  all  cases  the 
typical  jasper-bearing  Ogishke,  and  this  ma}'  be  followed  over  the  areas 
outlined  on  the  maps  and  traced  with  almost  continuous  exposures  through 
to  West  Gull  Lake.  It  will  be  noted  that  we  have  here  the  two  phases  of 
Ogishke  conglomerate — that  known  as  the  typical  form,  consisting  of 
striking  red  jasper  pebbles  with  large  quantities  of  granite  and  greenstone, 
and  that  variety  which  consists  essentially  of  greenstone  pebbles  with  no 
jasper  pebbles  and  only  a  few  granite  pebbles — separated  from  each  other 
by  the  width  of  the  lake — about  half  a  mile.  Between  these  lies  a  syncline 
of  the  Knife  Lake  slates.  As  these  phases  of  the  conglomerate  are  followed 
to  the  east  the  distance  between  them  gradually  increases,  a  headland  con- 
sisting of  Ely  greenstone  and  granite  of  Saganaga  Lake  coiuing  in  between. 
These  conglomerates  are  evidently  the  same.  The  difference  in  petro- 
graphic  character  can  be  readily  explained  as  due  to  a  difference  in  the 
underlying  rocks  from  which  they  were  derived.  North  of  the  headland 
of  Ely  greenstone  and  granite  of  Saganaga  Lake  the  conglomerate  con- 
sists, to  a  great  extent,  of  jiebbles  of  granite  derived  from  the  gi-anite  of 


THE  LOWER  HUEONIAN.  321 

Saganaga  Lake  and  pebbles  of  greenstone  and  jasper.  The  jasper 
evidently  was  derived  from  masses  of  the  Soudan  formation  which  were 
presumably  infolded  in  the  Ely  greenstone.  This  was  not  present  in  large 
quantity  and  is  now  buried  under  the  sediments,  or,  as  the  result  of  erosion, 
it  has  all  been  removed  and  is  now  found  only  as  pebbles  in  these  sedi- 
ments; at  least  no  masses  in  situ  have  thus  far  been  found.  South  of  the 
above-mentioned  headland,  where  the  conglomerate  derived  from  the  Elr 
greenstone  is  penetrated  by  an  occasional  dike  of  the  Saganaga  granite,  we 
find  that  the  pebbles  are  predominantly  greenstone  with  only  an  occasional 
granite  pebble.  Jasper  is  also  wanting  here.  As  we  follow  these  two 
belts  westward  they  come  closer  and  closer  together,  and  petrographicalh^ 
they  also  approach  more  nearly  to  each  other  as  the  result  of  the  inter- 
mingling of  the  granite  pebbles  and  jasper  pebbles,  until,  b}^  the  time  we 
reach  the  first-mentioned  area  at  the  west  end  of  the  lake,  the  two  conglom- 
erates are  close  together  and  are  petrographically  the  same.  Clearly  this 
was  the  place  where  the  currents  mingled  the  debris  derived  from  the  granite 
on  the  north  side  and  the  greenstone  on  the  south  side  of  this  great  west- 
ward-projecting headland, 

The  relations  of  the  conglomerate  to  the  undei'lying  rocks  are  clearly 
shown  by  the  fact  that  they  consist  of  pebbles  from  these  underlying 
rocks,  and  this  relationship  can  be  seen  at  a  number  of  locations,  to  which 
reference  has  already  been  made  in  preceding  pages  (p.  268  et  seq).  The 
conglomerates  have  been  found  in  actual  contact  with  the  greenstone  and 
with  the  granite,  so  that  there  can  be  absolutely  no  doubt  as  to  their 
actual  relationship.  Such  a  conglomerate,  lying  between  the  Knife  Lake 
slates  and  the  great  greenstone  mass  forming  the  Twin  Peaks  range  south 
of  Ogishke  Muncie  Lake,  was  described  by  N.  H.  Winchell." 

The  location  of  this  conglomerate  could  not  be  determined  from 
Winchell's  statement,  but  the  contact  along  this  range  was  followed  out 
for  a  long  distance.  In  many  places  the  slates  were  found  in  contact  with 
the  greenstone.  Where  these  first  contacts  were  found  the  greenstone  was 
schistose,  arid  no  distinct  conglomerate  was  observed.  Eventually, 
however,  on  the  east  slope  of  the  prominent  northward-trending  hill  of 
this  greenstone,  at  a  point  840  paces  south  and  650  paces  east  of  the 
meander  corner  between  sees.  27  and  26,  T.  65  N.,  R.  6  W.,  the  greenstone 


«Geol.  and  Kat.  Hist.  Survey  of  Minnesota,  Fifteenth  Ann.  Rept.,  1886,  pp.  372-374. 
MON  XLV — 03 21 


322 


THE  VERMILION  IRON-BEARING  DISTRICT. 


is  found  with  about  9  feet  of  a  conglomerate  derived  from  it  overlying  it. 
Above  this  folloAv  well-banded  slates  and  graywackes.  At  another  point, 
875  paces  south  and  700  east  of  the  same  location,  is  another  contact 
between  the  greenstone  and  the  sediments.  Here  we  find  a  band  of  con- 
glomerate formed  of  poorly  rounded  greenstone  pebbles  which  grades  up 
by  rapid  alternation  of  conglomerate  beds  with  finer-grained  sediments 
into  the  normal  Knife  Lake  slates.     The  bands  of  fine-grained  conglomerate 

vary  in  thickness  from  a  few  inches  to 
3  feet,  the  thinner  ones  alternating  with 
bands  of  the  slates.  The  entire  gradation 
here  takes  place  within  a  distance  of  about 
10  paces  from  the  greenstone  on  one  side 
to  the  normal  slates,  without  marked  con- 
glomerate bands,  on  the  other.  The  strike 
of  the  beds  here  is  N.  60°  W.,  with  a  dip 
of  80°  to  the  north.  As  may  be  seen  from 
the  map,  the  strike  follows  very  closely 
the  outcrop  of  the  greenstone. 

(3n  the  northern  slope  of  the  green- 
stone ridge  which  forms  a  subordinate 
anticline  at  the  southwest  end  of  Ogishke 
Muncie  Lake,  another  contact  was  found 
between  the  Ogishke  conglomerate  and 
the  greenstone.  The  contact  occurs  on 
the  hillside  at  a  place  225  paces  south  and 
20  paces  west  of  a  point  on  the  shore 
opposite  and  just  south  of  the  west  end 
of  the  westernmost  island  shown  on  the 
map  (PI.  XVI,  atlas).  The  conglon^erate 
here  is  fine  grained  and  consists  of  greenstone  pebbles  with  occasional  jasper 
pebbles.  Tlie  conglomerate  is  coarsest  near  the  eruptive  greenstone  and 
grows  progressively  finer  northward,  evidenth'  grading  upward  into  the 
slates  that  occupj^  the  central  portion  of  Ogishke  Muncie  Lake. 

The  large  island  northeast  of  the  east  end  of  this  same  greenstone 
anticline  well  deserves  examination,  as  it  shows  the  relations  of  the  green- 
stone and  the  conglomerate.     The  large-scale  sketch  (fig.  20)  shows  the 


200  feet 


Fig.  20. — Sketch  showing  relationship  of  Ely  green- 
stone and  overlying  Ogishke  conglomerate  on 
island  in  Ogishke  Muncie  Lake. 


THE  LOWER  HUEONIAN.  323 

distribution  of  the  rocks.  The  conglomerate  occurs  both  north  and  south 
of  the  greenstone,  but  on  the  east  side  of  the  island  near  the  top  of  the  hill 
there  is  a  small  area  of  slate  and  conglomerate  which  is  clearly  a  part  of 
the  Ogishke  conglomerate  that  was  infolded  in  the  underlying  greenstone 
and  has  not  been  completely  removed  by  erosion. 

When  this  greenstone  anticline  which  occurs  at  the  southwest  end 
of  Ogishke  Muncie  Lake  and  also  on  the  island  just  described  is  followed 
eastward  along  the  south  side  of  the  lake,  it  is  found  to  disappear  for  a 
distance  of  about  one-third  of  a  mile,  it  having  plunged  down  under  the 
conglomerate,  which  has  not  been  eroded  deep  enough  to  show  the 
greenstone.  It  reappears  again  near  the  north-south  section  line  between 
sees.  27  and  26,  T.  65  N.,  R.  6  W.  Here  the  conglomerate  is  found 
in  contact  with  the  greenstone  on  the  north  slope,  and  extends  north  of 
the  greenstone  over  a  considerable  area  to  the  lake  shore.  South  of  the 
greenstone,  however,  no  conglomerate  was  found.  Near  the  west  end  of 
this  anticline  the  slates  seem  to  come  in  almost  immediately.  The  actual 
contact  between  the  two  was  wanting,  and  a  thin  conglomerate  may 
occur  in  the  depression  between  the  two.  This  same  greenstone  anticline 
continues  eastwai'd  and  comes  out  again  on  the  west  side  of  the  bay  of 
Ogishke  Muncie,  into  which  empties  the  stream  which  comes  from  Fox, 
Agamok,  and  Gobbemichigamma  lakes.  Conglomerate  is  here  again 
exposed  all  along  the  north  side  of  the  anticline,  whereas  on  the  south 
side  the  slates  come  up  very  nearly  in  contact  with  the  greenstone.  This 
southern  edge  of  the  greenstone  was  here  followed  for  a  considerable 
distance,  in  search  of  an  actual  contact  between  the  greenstone  and  sedi- 
ments, and  at  one  place,  just  before  the  greenstone  exposures  cease  and 
where  a  swampy  area  begins,  a  small  patch  of  conglomerate  was  found 
hanging  on  the  sotith  face  of  the  greenstone.  The  conglomerate  here  must 
be  very  thin,  as  the  slates  begin  again  a  few  feet  south  of  the  face  of 
the  greenstone  ledge.  If  the  map  (atlas,  PI.  XVI)  is  referred  to  it  will 
be  noted  that  south  of  these  small  greenstone  anticlines  bordering  the 
south  shore  of  Ogishke  Muncie  Lake  there  occurs  a  broad  area  of  the 
Knife  Lake  slates  which  continue  south  to  the  great  Twin  Peaks  green- 
stone anticline.  The  presence  of  this  great  breadth  of  slates  between  these 
anticlines,  and  the  fact  that  where  they  are  in  contact  with  the  greenstone 
there  is  but  a  very  thin  mass  of  conglomerate,  indicate  that  during  Lower 


324  THE  VERMILION  IRON-BEARING  DISTRICT. 

Huronian  time  this  area  was  occupied  by  a  comparatively  protected  sea, 
in  which  wave  and  current  action  was  not  \erj  strong',  and  in  which  the 
slates  were  deposited  with  very  subordinate  masses  of  conglomerate. 

On  the  south  slope  of  the  great  greenstone  anticline  lying  south  of 
West  Grull  Lake  and  Gull  Lake  contact  between  the  greenstone  and  the 
conglomerate  derived  from  it  can  be  found  almost  anywhere  if  one  follows 
the  boundary  line  closely.  The  conglomerate  is  in  most  places,  however, 
so  coarse  that  sedimentary  banding  is  comparatively  rare,  and  this  may 
account  for  Grant's  error  in  considering  it  a  volcanic  tuff."  The 
conglomei-ate  here  is  exposed  over  an  area  about  If  miles  wide,  and  this 
great  width,  as  well  as  the  coarseness  of  the  conglomerate  is  evidence  of  its 
great  thickness  at  this  place.  Moreover,  where  bedding  is  shown,  for 
instance,  on  the  hill  725  paces  north  of  the  northeast  bay  of  Paul  Lake, 
the  beds  are  found  to  dip  very  steeply  to  the  north.  Clearly  this  greenstone 
was  the  shore  of  a  great  headland  which  was  exposed  to  violent  wave 
action,  for  otherwise  such  a  coarse  and  thick  conglomerate  would  not  have 
been  formed.  The  same  statement  is  true  for  the  great  conglomerate  which 
occurs  to  the  north  of  this  headland,  and  which  consists  to  a  great  extent 
of  granite  derived  from  the  granite  of  Saganaga  Lake,  although  here  the 
cono'lomerate  is  not  so  thick  as  that  south  of  the  headland. 

THE    AGAWA    FORMATION    (IRON-BEARING). 
DISTRIBUTION    AND    EXPOSURES. 

At  several  places  in  the  eastern  part  of  the  Vermilion  district  there  is 
found  a  carbonate-bearing  and  jaspery  iron-bearing  formation  which  is  very 
intimately  associated  with  the  Knife  Lake  slates  and  is  reall)'  but  a  phase  of 
these.  This  formation  occurs  in  widely  separated  areas,  and  in  each  instance 
it  is  exposed  in  comparatively  small  masses;  consequently  it  is  not  possible 
to  assert  with  i)erfect  confidence  that  all  of  these  ferruginous  rocks  belong- 
to  exactly  the  same  horizon,  although  we  are  sure  that  they  are  very  nearly 
contemporaneous.  The  areas  in  which  it  occurs  in  the  United  States  are  so 
small  that  we  can  state  confidently  that  it  will  never  be  of  economic  impor- 
tance. The  formation  is,  however,  very  much  more  extensively  developed 
in  portions  of  Ontario  adjacent  to  the  Vermili(in  district  of  Minnesota,  and 


"Geology  of  the  eastern  end  of  the  Mesabi  h'on  range  in  Minnesota,  iiy  IT.  S.  Grant:  Engineers' 
Yearbook,  University  of  Minnesota,  1898,  p.  54. 


THE  LOWER  HURONIAN.  325 

it  may  be  that  ore  deposits  will  eventually  be  found  in  this  area,  although 
it  is  very  doubtful  whether  the  formation  carries  ore  in  paying-  quantit}^  at 
any  of  the  places  examined.  The  greater  part  of  the  data  for  the  descrip- 
tion of  this  formation  was  obtained  from  Canadian  areas,  in  which  a  brief 
reconnaissance  was  made  and  in  which  this  formation  is  best  developed.  It 
derives  its  name  from  Agawa  Lake,  where  it  is  well  exposed. 

Bistributton. — The  iron-bearing  formation  occurs  only  in  narrow  belts 
which  are  not  continuous  for  great  distances.  The  westernmost  occurrence 
is  on  the  portage  between  Wind  and  Moose  lakes, "^  and  the  exposures  here 
consist  of  thin  bands  of  iron  oxide,  chert,  and  jasper  interbanded.  The  for- 
mation is  again  found  on  Sucker  Lake,  and  extends  thence  northeastward 
into  Birch  and  Carp  lakes  Here  it  is  in  character  a  ferruginous,  carbonate- 
bearing  slate.  It  has  been  followed  into  Ontario  for  about  12  miles  in  a 
direction  a  little  north  of  east  through  the  string  of  lakes  which  lie  about 
1^  miles  north  of  the  international  boundary  on  Knife  Lake,  and  are 
known  from  west  to  east  as  That  Mans,  Agawa,  This  Mans,  and  The 
Other  Mans  lakes.  In  this  area  it  is  present  in  very  characteristic  devel- 
opment, and  consists  chiefly  of  bands  of  chert,  jasper,  and  iron  oxides,  with 
a  carbonate-bearing  chert  and  ferruginous  slate  in  very  subordinate 
quantity.  The  next  area  in  which  it  is  known  to  occur  is  on  the  northeast 
arm  of  Ogishke  Muncie  Lake  on  both  the  northwest  and  southeast  shores. 
Especially  on  the  southeast  shore  is  it  well  developed.  Here  it  is  the 
carbonate-bearing  phase,  like  that  which  occurs  on  Birch  and  Carp  lakes. 

In  a  traverse  made  southward  from  Knife  Lake  along-  the  section 
line  between  sees.  29  and  30,  T.  65  N.,  R.  6  W.,  at  about  200  paces  south 
of  the  lake  shore  there  is  a  high  ridge  of  Knife  Lake  slates  and  gray  wackes, 
and  the  finer-grained  slate  grades  directly  down  into  banded  slate  and 
jasper.  Only  a  short  distance  south  of  this  occurs  the  basal  greenstone  on 
which  rests  the  slate  series.  Here  jasper  bands  are  interlaminated  with  the 
bottom  part  of  the  slate  series  which  seems  to  correspond  to  the  iron-bearing 
formation  observed  on  the  Moose  Lake- Wind  Lake  portage,  and  at  the  other 
points  noted  above.  It  could  not  be  followed  to  the  east  and  west.  A  similar 
occurrence  of  jasper  bands  in  the  Knife  Lake  slates  has  been  noted  upon 
Pickle  Lake.''     These  occurrences  are  interesting  as  showing  the  existence 

aGeol.  and  Nat.  Hist.  Survey  cf  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  p.  278. 
&  Grant:  Ibid.,  pp.  440  and  460. 


326  THE  VERMILION  IRON-BEARING  DISTRICT. 

of  this  iron-bearing'  formation  at  a  number  of  places  in  the  area,  but  in  each 
instance  the  occurrence  is  so  small  that  no  attempt  has  been  made  to  show 
them  on  the  map.  ]\Iention  of  these  localities  is  made  under  the  heading 
"Interesting  localities,"  page  330. 

Exposures. — The  exposures  are  not  very  numerous,  but  by  means  of 
them  the  belts  may  be  confidently  traced  out  along  the  strike,  and  these  are 
shown  on  the  accompanying  map  oidy  as  far  as  they  have  been  followed. 

STRUCTURE. 

The  iron  formation  occurs  in  narrow  belts,  having  a  very  uniform 
strike,  but  the  beds  of  these  belts  show  varying  strike  and  dip  throughout 
their  extent,  indicating  that  the  formation  has  been  folded  to  a  greater  or 
less  extent.  The  greatest  amount  of  folding  was  noticed  in  that  portion  of 
the  iron-bearing  formation  tliat  occm-s  on  the  portag-e  between  Wind  and 
Moose  lakes.  At  this  place  the  jasper  is  extremely  crenulated  and  broken. 
In  some  places  this  folding  is  so  extreme  that  the  bands  have  been  fractured 
and  the  fi-agments  drawn  out  into  pebble-like  areas  having  no  apparent 
connection  with  the  adjacent  pieces;  in  others,  however,  the  thin  string-like 
ends  may  be  connected  with  other  laminae  of  jasper  whicli,  when  followed 
out,  thicken  and  grade  into  other  pebble-like  areas  of  jasper. 

On  the  divide  on  the  Wind  Lake-Moose  Lake  portag'e  the  iron 
formation  is  exposed  in  three  different  belts,  all  tln-ee  essentially  parallel 
in  trend.  These  are  exposed  only  for  a  short  distance  along  their  strike. 
The  belts  have  very  much  the  same  appearance.  They  are  intensely 
plicated,  and  it  seems  from  close  study  that  they  all  belong  to  the  same 
horizon;  that  we  have  here,  in  other  words,  a  single  iron  formation  which 
was  intricately  folded  into  the  subjacent  conglomerates,  and  that  the  folds 
were  then  truncated,  making  the  formation  outcrop  at  three  different  places 
in  one  horizontal  section  so  as  to  look  like  three  superim])Osed  belts.  It 
is  confidently  believed  that  if  the  exposures  were  perfect  they  could  l)e 
followed  along  the  strike  and  connected  with  one  another. 

On  That  Mans,  Agawa,  This  Mans,  and  The  Other  Mans  lakes,  in 
Canada,  tlie  iron  formation  is  closely  t\)lded  into  a  syncline,  the  beds 
standing  practicalh'  on  edge.  Slates  occupy  the  center  of  the  syncline, 
and  in  going  away  from  this  center  in  both  directions,  north  and  south 
across  the  strike  of  the  l)eds,  one  goes  from  lower  to  lower  beds.      j\Iore- 


THE  LOWER  HURONIAN.  327 

over,  a  duplicate  succession  of  the  iron  formation  and  other  rocks  could  be 
determined,  showing  the  structure  to  be  synclinal. 

The  iron-bearing  formation  on  the  Canadian  lakes  is  locally  consid- 
erably folded,  but  in  no  cases  does  this  plication  reach  the  extreme  that  it 
does  in  the  Soudan  formation.  In  a  way  we  may  consider  the  plication 
of  the  rocks  as  a  measure  of  the  frequency  and  intensity  of  the  folding. 
Therefore  this  very  noticeable  difference  in  the  folding  of  the  two  iron- 
bearing  formations,  which  are  essentially  of  the  same  petrographic  character, 
is  indicative  of  the  lesser  age  of  the  one  under  consideration,  which  has 
been  assigned  to  the  Lower  Huronian. 

The  iron  formation  at  the  northeast  end  of  Ogishke  Muncie  Lake  is 
also  in  a  syncline,  the  formation  appearing  on  both  the  southeast  and  north- 
west shores  of  the  lake  The  formation  is  here  a  carbonate-bearing  slate 
and  is  especially  prominent  on  the  southeast  shore,  where  at  several  places  it 
occurs  in  high  cliffs  with  a  deep  brown  ocher-colored  crust.  When  this  is 
removed  it  discloses  a  clean,  white,  almost  pure  carbonate  rock  forming  the 
main  mass  of  the  ledge.  No  crumpling  could  be  detected  in  the  formation 
itself,  although  the  folding  of  the  formation  in  general  is  shown  by  the 
synclinal  structure  of  the  lake  basin. 

PETROGEAPHIC  CHAKACTEES. 

The  iron-bearing  Agawa  formation  consists  of  two  petrographic  facies, 
a  carbonate-bearing  slaty  facies,  and  a  chert,  jasper,  iron-oxide,  and  slate 
facies.  These  do  not  occur  together,  but  as  they  occupy  the  same  relative 
position,  at  the  base  of  the  Knife  Lake  slates,  they  are  supposed  to  belong 
to  the  same  horizon.  The  presumption  is  that  the  carbonate-bearing  facies 
is  of  essentialh^  the  same  kind  of  material  as  that  from  which  the  chert, 
jasper,  and  iron-oxide  facies  has  been  derived  as  the  result  of  processes 
of  metamorphism  similar  to  those  which  have  taken  place  in  the  production 
of  the  normal  jaspers  and  iron  oxides  from  the  ferruginous  cherts  in  the 
other  iron  formations  of  the  Lake  Superior  region  (p.  192). 

The  first  phase  of  the  iron-bearing  formation,  the  carbonate-bearing 
slates,  are  best  developed  on  the  southeastern  shore  of  Ogishke  Muncie 
Lake.  There  they  lie  at  the  base  of  the  Knife  Lake  slates,  resting  imme- 
diately upon  the  Ogishke  conglomerate,  which  has  been  derived  from  the 
subjacent  Ely  gi-eenstone.  These  carbonate-bearing  slates  have  been 
traced  along  the  southeast  shore  of  the  northeast  arm  of  this  lake  by  means 


328  THE  VERMILION  IRON-BEARING  DISTRICT. 

either  of  distinct  outcrops  or  of  marked  topographic  depressions  indicating 
their  continuation.  The  lake  here  lies  in  a  syncliue  in  the  sediments,  and 
on  the  opj^osite  shore  of  the  lake — that  is,  upon  the  northwest  shore — there 
are  carbonate-bearing  rocks  overlying  conglomerates  which  seem  to  repre- 
sent the  northwest  limb  of  this  syncline  and  to  be  a  repetition  of  the  car- 
bonates on  the  southeastern  shore.  In  the  rocks  on  this  northwest  shore  the 
carbonate-bearing  character  is  not  nearly  so  marked  as  in  those  on  the 
southeast  limb  of  the  syncline.  The  high  cliffs  of  the  carbonate  on  the 
southeastern  shore  have  been  noted  by  previous  observers,  and  have  been 
referred  to  by  Winchell  as  a  limestone.  This  rock  is  piil-e  enough  at  some 
of  the  exposures  on  the  southeast  shore  of  Ogishke  Muncie  Lake  to  be  called 
a  limestone.  Microscopic  examination  shows  that  it  consists  chiefly  of  a 
carbonate  with  a  small  amount  of  fine-grained  cherty  silica  and  some  cubes 
of  iron  pyrites.  The  carbonate  is  decidedly  ferruginous,  as  is  shown  by 
the  marked  vellow,  ocherous  weathered  crust.  It  passes  down  into  a 
carbonate-bearing  slate  and  then  into  the  Ogishke  conglomerate.  In  the 
other  direction,  upward,  it  passes  into  the  Knife  Lake  slates. 

This  carbonate-bearing,  horizon  is  assumed  to  be  the  representative  of 
the  cherty  iron  carbonate  rocks  from  which,  it  is  presumed,  the  iron-bearing 
rocks  at  the  same  horizon  at  other  places  were  derived. 

The  second  facies  of  the  iron-bearing  formation  of  the  Lower  Huro- 
nian  of  the  Vermilion  district  is  better  suited  to  bear  this  name  than  tlie 
carbonate-bearino-  rocks  just  described,  wdiich  contain  but  little  iron.  This 
second  facies  consists  of  chert,  jasper,  iron  oxide,  and  slate  interbanded. 
The  iron  oxide  is  chiefly  magnetite  with  very  little  hematite.  The  chert 
varies  from  white  to  gray  and  even  darker  when  it  has  more  magnetite  mixed 
with  it,  becomes  red  when  it  contains  hematite,  and  thus  passes  over  into 
the  brilliant-red  jasper.  This  jasper  is  relatively  rare  in  this  iron  formation. 
The  slates  are  the  normal  gray  to  bluish  and  greenish  slate  carrying  more 
or  less  ferruginous  carbonate,  like  the  above-mentioned  cherts.  The 
slates  are  very  much  changed,  and  their  clastic  characters  are  not  recog- 
nizable. They  are  now  ^-ery  fine-grained,  fissile,  well-banded  slates,  con- 
sisting of  small  chlorite  flakes,  grains  of  quartz,  some  ferruginous  calcite  in 
rhombohedra,  and  crystals  of  magnetite.  These  rocks  show  nothing  of 
especial  interest,  being  essentially  the  same  as  similar  rocks  forming  the 
iron-bearine-  Soudan  formation  of  the  Archean  and  hence  are  not  described 
here  in  detail. 


THE  LOWER  HUKONIAN.  329 


ORIGIN. 

The  uormal  iron-bearing  Agawa  formation — that  which  consists  of  the 
chert,  jasper,  and  iron  bands,  as,  for  instance,  on  the  Wind  Lake-Moose 
Lake  portage,  and  at  the  Canadian  locality  on  That  Mans,  Agawa,  This 
Mans  and  The  Other  Mans  lakes — is  identical,  so  far  as  its  petrographic 
character  is  concerned,  with  that  which  has  already  been  described  as  the 
Archean  Soudan  formation.  In  the  course  of  the  description  of  this  formation 
the  conclusion  was  reached  that  this  had  been  derived  as  the  result  of 
alteration — the  nature  of  which  was  also  discussed — from  an  original 
cherty,  iron-bearing  carbonate.  It  is  believed  that  this  iron-bearing 
Agawa  formation  was  derived  from  the  same  kind  of  rock,  and  as  the 
result  of  processes  analagous  to  those  by  which  the  Soudan  formation  was 
produced.  In  the  Soudan  formation  very  little  carbonate  was  found, 
the  reason  being,  very  evidently,  that  the  alteration  had  proceeded 
so  far  that  practically  all  of  the  carbonate  had  been  changed.  Some 
carbonate-bearing  bands  were  found  associated  with  the  jaspers  and 
cherts  on  That  Mans  Lake,  and  they  bear  a  striking-  resemblance  to  the 
carbonates  in  the  Soudan  formation.  The  carbonate-bearing  phases  of  the 
ii'on-bearing  Agawa  formation,  to  which  reference  has  been  made,  contain 
a  comparatively  high  percentage  of  iron,  as  is  shown  by  the  very  rich 
brown  ocherous  crust  which  is  found  wherever  the  rocks  have  been 
weathered.  It  is  believed  that  this  unaltered  carbonate-bearinor  horizon 
corresponds  to  the  jasperized  horizon,  and  that  these  unaltered  rocks  repre- 
sent an  earlier  jjhase  of  the  jasperized  rocks,  the  alterations  by  which  the 
jaspers  and  cherts  were  produced  not  having  taken  place  for  some  reason 
as  yet  unexplained. 

RELATIONS    TO    OTHER   FORMATIONS. 

In  the  j)receding  pages  statements  have  ah-ead}^  been  made  of  the  rela- 
tions which  these  rocks  bear  to  the  adjacent  formations,  and  the  details  will 
be  given  under  the  heading  "Interesting  localities." 

At  this  place  the  relations  of  the  iron  formation  to  the  adjacent  forma- 
tions will  be  concisely  stated.  In  the  first  place,  it  is  clear  that  the  iron 
formation  lies  above  the  Ogislike  conglomerate,  with  which  it  has  been 
found  in  contact  at  several  places.     In  every  instance  it  lies  above  the  con- 


330  THE  VERMILION  IRON-BEARING  DISTRICT. 

glomerate  and  between  it  and  the  overlying  Knife  Lake  slates,  into  which 
at  other  places  the  conglomerate  grades.  While  this  foi-mation  is  not  found 
everywhere  between  the  conglomerates  and  slates,  in  those  places  where  it 
does  occur  it  always  occupies  that  position.  It  is  clearly,  then,  an  inter- 
mediate horizon  of  comparatively  local  origin,  and  our  studies  have  shown 
that  in  the  Vermilion  district  it  is  unimportant  from  an  economic  standpoint. 

AGE. 

In  age  it  is  therefore  younger  than  the  Ogishke  conglomerate,  and  older 
than  the  great  mass  of  Knife  Lake  slates,  and  forms  a  part  of  the  Lower 
Huronian  series. 

THICKNESS. 

The  thickness  of  the  formation  varies  considerably.  On  the  Wind 
Lake-Moose  Lake  portage  it  has  a  thickness  of  about  6  feet.  At  other 
places  on  the  United  States  side  of  the  boundary  the  exposures  are  so  poor 
that  nocon-ect  determinations  could  be  made  of  its  thickness,  but  at  all  of 
these  places  it  appears  to  be  considerably  thicker  than  at  the  locality  just , 
mentioned.  The  best  opportunity  for  determining  its  thickness  was  aiforded 
by  the  exposures  in  Canadian  territory,  where  very  accurate  determination 
could  be  made.  The  best  exposures  seen  in  this  area  are  those  on  the  range 
of  hills  crossed  by  the  portage  from  Agawa  Lake  into  This  Mans  Lake. 
The  rocks  are  here  exposed  in  a  syncline,  and  on  each  side  of  the  center 
of  this  syncline,  which  is  occupied  by  a  belt  of  slates,  there  was  found  the 
alternating  series  of  belts  of  jasper  and  slates  forming  the  iron-formation 
complex  and  having  a  total  thickness  of  about  50  feet. 

INTERESTING   LOCALITIES. 

The  best  place  at  which  to  stud)-  the  characters  of  the  Agawa  forma- 
tion is  on  the  string  of  lakes  known  as  That  Mans,  Agawa,  This  Mans, 
and  The  Other  Mans  lakes,  which  lies  at  an  average  distance  of  about 
1^  miles  north  of  the  international  boundary  on  Knife  Lake,  and  trends 
about  N.  45°  E.  Exposures  of  the  formation  may  be  seen  at  intervals 
along  the  south  shore  of  the  string  of  lakes,  on  the  islands  near  the 
center,  and  on  the  necks  of  land  which  separate  the  lakes  and  across 
which  the  portages  run.  On  the  point  north  of  the  north  end  of  the 
portage  that  comes  from  Emerald  Lake  on  the  south  to  That  ]\Iaus  Lake 


THE  LOWER  HURONIAN.  381 

there  is  a  series  of  interbedded  jaspers,  cherts,  iron  ore,  carbonate-bearing 
slates,  and  normal  slates  and  graywackes,  having  a  width  of  about  50  feet. 
The  dips  on  this  exposure  are  all  to  the  south.  No  dips  were  taken  which 
were  less  than  55°,  and  most  commonly  they  were  60°.  On  the  southeast 
side  of  the  bay  northeast  of  this  point  there  is  an  excellent  exposure  which 
gives  a  cross  section  of  the  iron  formation.  The  best  exposures  in  this 
area,  however,  are  those  on  the  range  of  hills  crossed  by  the  portage  from 
Agawa  Lake  into  This  Mans  Lake.  On  the  hill  immediately  north  of 
the  west  end  of  the  portage  there  is  well  exposed  a  series  of  narrow 
interlaminated  bands  of  jasper,  iron  oxide,  chiefly  magnetite  and  chert,  with 
some  slaty  bands.  Between  these,  and  interlaminated  with  them,  there 
occurs  well-banded  and  thinly  laminated  gray  slate.  These  interbanded 
belts  of  slates  and  iron  formation  proper  continue  to  outcrop  along  the 
hill  to  the  east.  From  this  hill  a  north-south  traverse  was  made,  and  here 
it  was  found  that  the  youngest  rock,  that  which  occupies  the  center  of  the 
area,  is  a  gray  sericitic  slate  having  a  width  of  about  75  feet,  and  striking 
about  N.  45°  E.  On  each  side  of  this — that  is,  both  north  and  south  of  it — 
there  occurs  the  iron  formation,  consisting  of  a  complex  of  three  belts  of  the 
iron  formation  proper,  and  two  intervening  belts  of  slate.  The  approximate 
width  of  the  complex  on  each  side  of  the  central  belt  of  slates  is  50  feet. 
North  and  south  of  the  iron  formation  there  is  a  considerable  width  of  gray 
slates,  which  are  in  their  turn  succeeded  by  intei'bedded  graywackes  and 
slates,  and,  finallv,  north  and  south  of  these  sediments  comes  the  Elv  g-reen- 
stone  as  basement.  This  greenstone  was  not  seen  at  just  this  locality,  but 
its  relation  to  the  sediments  was  obtained  on  the  strike  of  these  beds  to  the 
west,  and  but  a  short  distance  away  from  this  point.  From  the  repetition 
of  these  various  rocks  it  is  clear  that  the  structure  at  this  place  is 
synclinal.  The  greenstones  on  the  south  and  north  represent  the  oldest 
rocks,  the  younger  sediments  occurring  above  them  toward  the  center 
of  the  syncline.  The  structure  of  the  rocks  evidently  determined  the 
topography  in  this  region.  The  lakes  lie  in  the  center  of  and  at  the  bottom 
of  this  slate  syncline.  It  is  true  that  at  this  place  the  iron  formation  does 
not  lie  absolutely  at  the  base  of  the  slates,  since  there  are  some  slates 
between  it  and  the  conglomerate.  Nevertheless  it  occurs  essentially  at  the 
base  of  the  formation.  This  was  an  area  in  which,  as  is  shown  by  the 
rocks,  the  conditions  of  deposition  were  rapidly  changing.     Slates  were  at 


332  THE  VERMILION  IRON-BEARING  DISTRICT. 

first  formed,  and  as  the  conditions  changed  they  graded  into  the  rocks 
forming  the  iron  formation,  which  in  their  tnrn,  as  the  conditions  again 
changed,  graded  again  upward  into  normal  slates.  This  was  repeated  at 
least  three  times.  On  good  exposures  the  belts  of  iron  formation  proper 
are  made  up  predominantly  of  jasper,  iron  oxide,  and  chert,  but  here  and 
tliere  slate  bands  are  present,  and  on  the  sides  of  such  a  belt  the  slate 
gradually  increases  in  quantity,  jasper  and  ore  gradually  diminishing,  until 
we  pass  into  a  belt  of  finel}^  laminated  slate  shiowing  on  the  weathered 
surfaces  alternating  pink,  white,  and  greenish  bands,  with  which  there  is 
jDractically  lao  jasper. 

The  carbonate-bearing  rocks  occur  at  various  places  on  Birch  Lake. 
They  were  always  found  along  the  contact  between  the  greenstones  and 
the  slates,  and  were  a  sure  guide  to  the  close  proximity  of  the  greenstones. 
No  jasper  was  found  with  these  rocks.  Essentially  the  same  conditions 
prevail  on  the  north  side  of  Carp  Lake.  At  a  number  of  places  here  the 
belt  of  ferruginous  carbonate-bearing  slate  was  found  between  the  green- 
stones and  the  slates.  At  one  place  a  series  of  bands  of  chert  and  jasper 
was  observed.  These  occur  at  the  west  end  of  the  north  arm  of  Carp 
Lake,  on  the  little  point  just  opposite  an  exposure  of  the  Ely  greenstone. 
They  appear  very  similar  to  the  slates  and  jasper  that  occur  in  the  series 
at  This  Mans  Lake,  northeast  of  this  place.  The  greenstone  shows  its 
typical  ellipsoidal  characters  and  is  separated  from  the  jasper  by  an  interval 
of  2  feet.  This  occurrence  corresponds  very  closely  to  that  on  the  north 
flank  of  the  small  greenstone  anticline  south  of  Knife  Lake,  in  sees.  29  and 
30,  T.  65  N.,  R.  6  W. 

One  can  readily  see  how,  in  the  course  of  the  deposition  of  slates  of 
such  great  thicknesses  as  those  that  make  up  the  Knife  Lake  slates  of  the 
Lower  Huronian  in  the  Vermilion  district,  there  could  occiu-  at  different  times 
conditions  favorable  for  the  deposition  of  rocks  from  which  might  be  derived 
materials  similar  to  the  iron-bearing  formation.  We  would  thus  expect  to 
find  liere  and  there  throughout  these  slates  rocks  essentially  similar  to  those 
of  the  iron-bearing  formation.  Li  this  way  we  may  account  for  the  occur- 
rence in  the  slates  west  of  Ely,  near  the  east  quarter  post  of  sec.  4,  T.  62  K., 
R.  13  W.,  of  a  series  of  alternating  chert  and  cherty-slate  layers  bearing 
some  iron.  The  less  cherty  layers  contain  a  considerable  quantity  of  iron, 
and  weather  Avitli  fairly  brilliant  red  color.     In  this  way, .by  the  alternation 


THE  LOWER  HURONIAN.  333 

of  the  red  bands  and  the  gray  chert  bands,  these  rocks  simulate  the  jaspers 
and  cherts  of  the  iron  formation  pi'oper.  A  somewhat  similar  occurrence  is 
that  noted  at  the  south  end  of  the  portage  coming  into  the  north  side  of 
Pickle  Lake,  where  there  are  alternating  slate  and  pui-plish  chert  bands, 
striking  N.  35°  E.  in  the  midst  of  the  Knife  Lake  slates.  These  are 
probably  the  continuation  of  similar  chert  bands  which  occui-  upon  the 
southeast  shore  of  Pickle  Lake. 

On  the  bare  hills  crossed  by  the  Wind  Lake-Moose  Lake  portage 
tnere  are  good  exposures  of  the  iron-bearing  Agawa  formation,  showing 
its  relationship  to  the  adjacent  rocks.  To  the  north  is  a  conglomerate 
consisting'  of  greenstone  pebbles  with  occasional  pebbles  of  acid  rocks,  feld- 
spathic  porphyry,  rhyolite-porphyry,  and  granite.  Above  the  conglomerate 
is  about  3  feet  of  coarse  graywacke  and  fairly  coarse  slate,  followed  by  a 
belt  about  a  foot  and  a  half  in  thickness,  in  which  the  interbanding  is  closer 
with  the  slate  predominating.  Then  comes  3  feet  of  the  iron-formation 
member,  consisting  chiefly  of  the  black  chert — black  jasper  as  it  is  some- 
times called — greenish  chert,  and  some  red  jasper,  although  this  is  in  rather 
subordinate  quantity.  With  these  cherts  there  are  found  a  few  fine-grained 
slaty  layers  ranging  in  thickness  from  a  fraction  of  an  inch  to  4  inches. 
This  band  of  the  iron-bearing  formation  is  6  feet  wide.  South  of  this  iron- 
bearing  formation,  and  making  up  the  remainder  of  the  section  of  this  hill, 
and  exposed  for  a  hundi-ed  feet  or  more,  across  the  strike,  is  a  conglomerate 
similar  to  the  conglomerate  below  the  jasper  band  in  its  essential  characters, 
but  different  from  it  in  several  important  respects:  First,  it  contains  several 
narrow  but  minutely  crenulated  complex  layers  of  interlaminated  gray- 
wacke, slate  and  jasper;  second,  it  contains  many  roundish,  lenticular,  and 
also  angular  areas  of  black  chert,  red  jasper,  and  a  black  slaty-looking 
jasper,  and  various  combinations  of  these.  When  these  areas  are  examined 
closely,  they  appear  to  be  true  beds  which  have  been  broken  by  dynamic 
action.  The  undoubted  jasper  formation  itself  is  extremely  crenulated  and 
broken.  Generallj^  the  pebble-like  areas  continue  out  into  thin  strings 
which  may  be  connected  with  laminae  of  jasper,  but  this  is  not  invariably 
the  case.  If  the  jasper  is  in  true  fragments  the  slate  and  graywacke  also  are 
in  true  fragments,  for  they,  too,  occur  in  this  material  in  similar  roundish  or 
angular  masses.  It  has  been  suggested  that  this  conglomerate  lying  above 
the  jasper  belt  first  described  is  younger  than  the  jasper,  and  that  these 


334  THE  VERMILION  IRON-BEARING  DISTRICT. 

in-egular  masses  which  have  just  been  mentioned  are  pebbles  derived  from 
it  and  lying  in  the  conglomerate.  While  one  can  understand  how  this 
interpretation  could  be  made,  the  conglomerate  above  the  jasper  baud  being 
regarded  as  evidence  of  another  structural  break,  nevertheless  the  undoubted 
l^ands  of  jasper  in  the  conglomerate,  and  the  certainty  that  many  of  these 
irreo'ular  areas  are  derived  from  these  bands  and  owe  their  character  to 
dynamic  agencies,  are  strong  evidence  against  this  view.  It  may  be  sug- 
gested that  it  is  very  probable  that  some  of  these  irregular  masses  may  be 
due  to  a  later  infiltration  of  jasper  material. 

The  interbanding  of  the  jasper,  slate,  and  conglomerate  is  particularly 
well  seen  about  50  paces  west  of  the  trail  near  the  top  of  the  hill.  Here 
in  a  width  of  6  feet  one  may  count  6  distinct  bands  of  jasper  interbedded 
with  fine  graywacke  and  slate.  The  bands  which  were  counted  as  jasper 
bands  contain  thin  laminge  of  slate  ranging  from  a  fraction  of  an  inch  to  an 
inch  across,  and,  vice  versa,  the  bands  counted  as  clastic  sediments  contain 
minute  bands  of  jasper.  At  this  place  the  extreme  mashing  to  which  the 
rocks  have  been  subjected  in  this  area  is  beautifully  illustrated.  The 
jasper  is  plicated  in  an  extremely  complex  manner.  In  some  places  it 
bends  without  major  fractures,  and  in  others  it  has  broken  through  and 
through.  In  places  narrow  bands  of  the  jasper  are  severed  by  diagonal 
shearing  planes  into  areas  which  are  now  more  or  less  lenticular  in  shape, 
and  may  be  immediately  in  juxtaposition  or  somewhat  removed  from  one 
another.  In  the  clastic  sediments  lying  between  the  continuous  jasper 
bands  there  are  some  angular  and  roundish  areas  of  jasper,  but  these 
appear  to  have  been  derived  by  djmamic  action  from  the  continuous  belts 
of  jasper,  and  not  to  be  clastic  fragments  deposited  b}'  water  in  their 
jDresent  place.  The  whole  is  made  more  complex  by  secondary  veining 
and  jasperization  of  the  rocks  since  the  folding  of  them  took  place.  South 
of  the  last-mentioned  jasper  belt,  there  occurs,  measured  across  the  strike, 
about  50  feet  of  graywacke,  slate,  and  conglomerate.  On  the  south  side  of 
the  exposure  this  material  changes  into  a  fine-grained  conglomerate  which 
in  the  space  of  3  or  4  feet  grades  up  into  a  slate  which  underlies  the  150  to 
200  paces  intervening  between  the  last  jasper  exposure  on  the  ridge  and 
Moose  Lake.  A  close  examination  of  these  jasper  bands  on  the  hills  sliows 
that  the  second  one  to  the  south,  which  lies  between  two  ridges  of  con- 
glomerate, is  of  just  about  the  same  width  as  the  jasper  band  which  appears 


THE  LOWER  HURONIAN.  335 

farthest  north.  This  suggests  that  the  double  appeai-a.nce  of  this  band 
is  due  to  infolding,  the  closeness  of  which  is  evidenced  by  the  extremelj 
plicated  character  of  the  iron  formation  itself  East  of  this  ridge,  in  the 
low  ground,  the  iron  formation  was  found  exposed,  but-  so  poorly  that  it 
was  impossible  to  determine  its  width.  It  is  followed  to  the  south  by 
slates,  whereas  the  conglomerates  are  the  nearest  rocks  exposed  to  it  on  the 
north.  It  seems  almost  unquestionable  that  we  have  here  an  iron-bearing 
formation  of  very  limited  thickness,  which  occupies  a  horizon  between 
the  conglomerates  north  of  and  below  it,  and  the  younger  slates  soiith  of 
and  above  it.  This  iron  formation  could  not  be  traced  to  the  east  or 
the  west  of  the  area  mentioned.  At  one  place,  however,  on  the  bare 
hill  in  tlie  SE.  J  of  sec.  16,  T.  64  N.,  R.  9  W.,  overlooking  the  swamp 
which  runs  down  to  the  southwest  end  of  Newfound  Lake,  occur  o-ood 
exposures  of  a  ver}^  feldspathic  fragmental  rock.  This  coarse  fragmental  is 
interbanded  with  fine-grained  slates.  Here  were  found,  in  a  few  places, 
interlaminated  with  the  slates,  narrow  bands  of  black  chert  up  to  3  inches 
in  width.  This  chert  is  conformable  with  the  slates,  and  seems  to  be 
contemporaneous  with  them,  but  may  possibly  consist  of  silicified  lenses 
of  carbonate-bearing  rock.  These  bands  disappear  after  being  followed 
for  only  very  short  distances.  The}^  occur  fairly  close  to  the  greenstone 
to  the  northwest,  on  which  rest  these  sediments  in  unconformable  relations 
without  intervening  thick  masses  of  conglomerate. 


KNIFE  LAKE   SLATES. 


One  of  the  largest  lakes  on  the  canoe  route  along  the  international 
boundary  has  been  known  since  the  time  of  the  fur  traders  as  Knife  Lake 
(Lac  des  Couteaux).  The  lake  was  so  named  because  of  the  flinty  con- 
choidally  fracturing  slates  which  surround  it  and  which,  with  their  sharp 
knife-like  edges,  cause  a  great  deal  of  inconvenience  to  the  moccasined 
traveler,  and  even  to  him  with  thick  boots.  Probably  this  name  is  but  a 
translation  of  one  given  to  it  by  the  Indians  long  before  the  advent  of 
the  French  voyageurs.  At  any  rate,  the  name  aptly  describes  one  of  the 
characteristics  of  the  slates,  and  the  lake  bearing  the  name  is  so  prominent 
a  featui-e  of  the  hydrography  of  the  district  that  the  name  has  been  given 
also  to  the  slates. 


336  THE  VERMILION  IRON-BEARING  DISTRICT. 

PETKOGRAPHIC    CHARACTERS. 

Macroscopic  characters  — In  mapping  the  Knife  Lake  slates  it  has  been 
found  desirable  to  include  with  them  numbers  of  beds  of  grit  and  fine- 
gi'ained  conglomerates  which  belong  structurally  with  them.  These,  how- 
ever, are  so  unimportant  relative  to  the  great  mass  of  the  slates  that  they 
will  not  be  considered  in  the  further  description  unless  they  possess  especial 
characters  that  warrant  reference  to  them.  The  slates  vary  from  those 
which  are  exceedingly  fine  grained  and  aphanitic  to  grits.  As  a  rule,  the 
exceedingly  fine-grained  forms  predominate.  They  are  for  the  most  part  not 
eartliv  clay  slates,  but  are  flinty,  break  with  a  ringing  sound,  have  con- 
choidal  fracture,  and  form  fragments  with  sharp,  cutting-  edges.  The  normal 
clay  slates  are  in  decidedly  smaller  quantity  than  the  above-mentioned 
flinty  forms.  The  color  of  the  slates  is  in  general  rather  dark  on  fresh  frac- 
ture, varying  from  dark  gray  and  olive  green  to  bluish  black.  Associated 
with  these  dark  slates  are  light-grayish  and  greenish-colored  slates  and 
graywackes.  Occasional  bands  of  white  to  gray  and  purplish  black  cherts 
occur  with  the  slates.  On  weathered  surfaces  the  normal  slates  have  a  light 
gray  to  light-brownish  color.  The  flinty  slates,  however,  weather  with  an 
almost  snow-white  crust,  showing  maci'oscopically  by  this  weathering  that 
they  consist  to  a  very  considerable  extent  of  silica  in  an  exceedingly  fine 
state  of  division.  This  is  in  strong  contrast  to  their  invariable  dark  bluish- 
black  color  on  fresh  fracture. 

In  general,  there  is  a  difference  iii  the  slates  in  different  portions  of  the 
district,  due  primarily  to  the  character  of  the  rocks  from  which  they  are 
derived.  As  a  rule,  where  the  Knife  Lake  slates  and  graywackes  lie  next  to 
the  Archean  greenstones,  without  large  masses  of  Ogishke  conglomerate 
between  them,  they  are  green,  and  grade  into  the  normal  gra}-  and  blue 
Knife  Lake  slates,  made  up  in  large  proportion  of  granitic  debris  only  at 
considerable  distance  from  the  greenstones.  Slates  of  this  greenish  color 
are  especially  noticeable  on  Birch,  Carp,  and  Ensign  lakes,  and  in  the  area 
southwest  of  Emerald  Lake.  From  Moose  Lake  east  to  Ensign  Lake  the 
slates  range  from  the  greenish  ones,  looking  much  like  extremely  fissile 
green  schists,  to  sericitic,  gray,  fissile  slates,  which  predominate.  IS  ortheast 
of  Ensign  Lake  the  slates  again  grade  into  the  green  ones,  and  in  Bass 
Lake  the  normal  Knife  Lake  slates  are  at  some  places  very  flinty,   and 


THE  LOWER  HURONIAN.  337 

at  others  have  white  weathered  surfaces.  There  occur  with  the  slates  at 
times  graywackes  which  are  i-ather  puzzhng.  They  are  more  intimately 
associated  with  the  slates  than  with  the  conglomerates,  and  hence  will  be 
described  in  this  place.  South  of  the  east  end  of  Moose  Lake,  and  in  fact 
at  a  number  of  places  on  the  hills  south  of  this  lake,  especially  in  the 
viciiaity  of  the  portage  from  Moose  Lake  to  Flask  Lake,  there  occurs  a 
graywacke  which  has  a  greenish  color,  and  from  its  green  background 
there  stand  out  very  prominent  rounded  porphyritic  feldspars.  The 
appearance  is  very  similar  to  that  of  a  feldspathic  porph3^ry  which  occurs 
in  the  immediate  Adcinity  of  these  graywackes.  Discrimination  between 
the  two  is  difficult.  In  fact,  there  is  evidently  a  gradation  between  them, 
the  graywacke  representing  the  disintegrated  jDortion  of  the  porphyry,  the 
particles  of  which  have  been  but  slightly  moved  and  worn.  Bedding  is  not 
very  distinct  in  it.  It  is  now  very  schistose,  and  one  can  trace  the  passage 
from  this  schistose  graywacke  into  the  fairly  massive  poiphyry. 

Somewhat  similar  feldspathic  sediments  with  the  feldspars  appearing 
almost  like  phenocrysts  occur  on  the  bare  hill  in  the  SE.  ^  of  sec.  16, 
T.  64  N.,  R.  9  W.,  just  west  of  the  southwest  end  of  Newfound  Lake. 
These  occurrences  correspond  very  closely  to  those  graywackes  which 
occur  at  Vermilion  Lake  (p.  288)  in  immediate  contact  with  the  various 
acid  inti-usives,  and  which  in  many  cases  can  not  be  distinguished  from 
them  in  the  field. 

Microscopic  characters. — The  microscopic  examination  reveals  nothing 
of  especial  interest.  The  essential  primary  constituents  are  feldspar,  quartz, 
brown  mica,  white  to  green  and  violet-brown  pyroxene,  and  greenish-brown 
hornblende,  and  then  there  is  always  an  amount  of  the  fine  interstitial 
material,  the  very  fine  product  of  the  attrition  of  the  grains  of  minerals  and 
fragments  of  rock  forming  the  slates  and  graywackes.  This  material  in  all 
cases  studied  has  been  recrystallized,  and  does  not  show  up  now  as  a  dark 
interstitial  mass  except  by  low  power.  By  high  power  the  individual  con- 
stituents can  usually  be  recognized  and  will  be  referred  to  below.  In  the 
graywackes  which  are  associated  with  the  slates  there  occur  occasional 
minute  fragments  of  the  various  rocks  which  have  been  mentioned  in 
previous  pages  as  forming  a  part  of  the  conglomerates.  The  primary  min- 
eral grains  and  likewise  the  interstitial  dust  have  very  frequently  been 
extensively  altered,   and   from   these  have    been  produced   the   following 

MON  XLV— 03 22 


338  THE  VERMILION  IRON-BEARING  DISTRICT. 

secondary  minerals,  which  in  some  cases,  where  the  rocks  are  completely 
recrystallized,  are  the  sole  constituents:  Chlorite,  epidote,  sericite,  actinolite, 
massive  dark-brown  and  gi'een  hornblende,  quartz,  calcite,  and  pyrite.  The 
material  between  the  grains  in  the  coarser  sediments  is  made  up  of  sericite, 
chlorite,  epidote,  quartz,  and  feldspar,  and  this  is  believed  to  have  been 
produced,  as  stated  above,  from  the  fine  detrital  material  originally  lying 
between  these  grains.  In  the  very  fine-grained  rocks,  where  the  crystalline 
character  of  this  interstitial  material  is  recognizable,  but  where  the  indivi- 
duals of  it  could  not  be  determined,  they  are  presumed  to  be  the  same  as 
those  just  enumerated.  These  materials  are  present  in  varying  proportion, 
which  produces,  of  course,  the  difiFerences  in  color  and  chemical  composition 
of  the  rocks;  for  instance,  some  of  the  lighter  colored  rocks — the  light 
cherts,  for  example — are  composed  essentially  of  quartz  in  very  fine 
crystalline  particles.  Other  rocks  are  made  up  essentially  of  quartz  and 
feldspar  in  waterworu  grains,  with  some  of  the  fine  interstitial  material 
between.  Othei-s  again  contain,  in  addition  to  the  quartz  and  feldspar,  a 
large  quantity  of  pyroxene  and  of  hornblende,  and  are  very  dark,  usually 
dark  green,  and  apparently  fairly  basic  in  character.  A  few  rocks  contain 
calcite  in  considerable  amount,  but  never  in  sufficient  amount  to  be  called 
limestone.  Moreover,  this  calcite  is  believed  to  be  of  secondary  origin, 
derived  from  the  alteration  of  the  minerals  forming  the  rocks  or  else 
introduced  from  other  sources  by  infiltration. 

The  general  alteration  which  the  minerals  of  these  rocks  have  already 
uuderg'one  has  been  referred  to.  The  addition  of  new  minerals  is  cleai'ly 
shown  in  the  case  of  the  hornblende  of  some  specimens.  In  these  we  find 
the  fragments  of  dark-brownish  hornblende  surrounded  hj  recently  added 
massive  light-greenish  hornblende.  The  hornblende  grains  in  some  cases 
have  been  increased  to  two  and  even  three  times  their  original  length." 

The  changes  which  have  taken  place  in  some  of  the  rocks  show  ver}^ 
well  how,  by  somewhat  further  changes,  banded  crystalline  schists  might 
be  produced  which  would  offer  no  clue  but  the  banding  to  a  determination 
of  the  kinds  of  rocks  from  which  they  were  derived.  Thus  some  of  the 
fine-grained  sediments,  rich  in  hornblende,  have  nearly  all  of  the  pieces 
of  hornblende,  very  commonly  cleavage  pieces,  arranged  with  their  long 

"  Enlargements  of  hornblende  fragments,  by  C.  R.  Van  Hise:  Am.  Jour.  Sci.,  3d  series,  Vol.  XXX, 
1885,  pp.  231-235. 


THE  LOWER  HUEONIAN.  339 

directions  parallel.  Mauy  of  these  have  been  added  to  by  secondary 
growths  of  massive  hornblende  or  of  fibrous  actinolite.  This  addition  of 
the  secondary  material  only  tends  to  emphasize  the  parallel  arrangement 
of  the  jDarticles,  since  the  greatest  addition  has  been  made  on  the  ends  of 
the  fragments.  This  was  noticed  in  some  of  the  graj'-wackes  made  up 
almost  exclusively  of  feldspar  and  hornblende,  with  mica  next  in  abun- 
dance, and  very  little  quartz.  In  such  rocks  the  biotite  alters  to  chlorite. 
Alteration  of  the  feldspar  with  the  production  of  new  minerals  also  takes 
place,  and  a  few  quartz  grains  alone  remain  to  give  evidence  of  the  clastic 
character  of  the  constituents.  When  such  rocks  take  part  in  orogenio 
movements  the  quartz  may  be  crushed,  and  in  general  a  better  degree  of 
schistosity  produced  than  previously  existed,  and  as  a  result  of  such  move- 
ments hornblende-schists  may  eventually  be  produced.  The  banding  of 
these  hornblendic  sediments  has  been  referred  to.  These  bands  of  finer 
and  coarser  materials  have  essentially  the  same  composition.  There  also 
appears  to  be  a  relation  between  the  fineness  of  the  hornblende  grains  of 
the  original  sediments  and  the  character  of  the  new  amphibole.  Thus  it 
was  noticed  that  in  tlie  alteration  of  the  very  fine-grained  sediments  the 
new  amphibole  is  added  in  the  form  of  fine  actinolite  needles,  whereas  in 
the  coarser  graywackes  the  new  amphibole  is  in  general  a  massive  horn- 
blende. The  final  pi'oduct  of  the  metamorphism  of  these  rocks  would 
probably  retain  this  essential  difi"erence,  so  that  we  would  g-et  fine-  and 
coarse-grained  ampliibole-schists,  or  possibly  banded  actinolite  and  horn- 
blende-schists. 

There  is  also  a  banding  due  to  the  separation  of  the  kinds  of  minerals. 
Thus  there  will  be  some  dark  Ijands  made  up  essentially  of  hornblende  and 
mica  with  but  little  feldspar,  and  alternating  with  these  bands  there  will 
be  bands  of  feldspar  with  but  little  hornblende  or  biotite.  When  such 
rocks  are  metamorphosed  there  will  be  no  very  great  migration  of  material 
from  the  one  band  to  the  other,  and  certainly  the  original  differences  in  the 
bands  will  be  shown  to  a  certain  extent  in  the  differences  in  the  bands  in 
the  metamorphic  product.  Presumably  the  final  product  would  be  an 
amphibole-schist  complex  made  up  of  alternating  bands  of  amphibole- 
schists  rich  and  poor  in  feldspar  or  other  secondary  products — quartz  and 
feldspar,  perhaps — derived  from  the  original  feldspar  and  of  different  color, 
corresponding  to  the  difference  in  mineral  composition. 


340  THE  VERMILION  IRON-BEARING  DISTRICT. 


METAMORPHISM    OF   THE   KNIFE    LAKE    SLATES. 

The  rocks  possessing'  the  characters  briefly  described  above  are  those 
which  we  may  call  the  normal  Knife  Lake  slates.  Even  these  so-called 
normal  slates  have  been  very  much  metamorphosed.  The  metamorphism 
has  been  caused  by  the  infiltration  of  material  (chiefly  silica  and  calcium 
carbonate),  by  the  cementation  of  the  particles  by  these  substances,  and 
hj  the  further  cementation  of  the  rock  by  chemical  changes  of  the 
fragments,  which  produced  new  minerals  and  also  caused  the  secondary 
enlargement  of  the  old  mineral  fragments.  These  rocks  show  locally  the 
effect  of  orogenic  movements  in  more  or  less  well-developed  schistose 
structure  and  fracturing. 

At  several  places  in  the  district  the  slates  are  in  contact  with  later 
igneous  rocks,  both  acid  and  basic,  and  have  been  metamorphosed  by  the 
intrusion  of  these  rocks.  In  all  cases  the  rocks,  as  stated  above,  have 
been  affected  by  processes  of  cementation  and  to  a  certain  extent  by 
orographic  movements,  and  in  some  instances  these  sediments  have  been 
metamorphosed  by  acid  intrusives  and  then  have  been  acted  upon  by  the 
great  Keweenawan  gabbro  and  still  further  changed.  Hence  it  is  exceed- 
ingly diflicult  to  discriminate  between  the  kinds  of  metamorphic  products 
wliich  are  due  to  each  one  of  these  agencies.  In  the  following  paragraphs 
a  brief  description  will  be  given  of  the  macroscopic  characters  of  those 
Knife  Lake  slates  which  have  been  metamorphosed  by  the  contact  action 
of  the  acid  and  basic  intrusives,  and  which  occur  in  the  four  most  important 
areas:  (1)  South  of  Tower,  along  the  Duluth  and  Iron  Range  Railroad,  near 
milepost  92;  (2)  on  the  Kawishiwi  River;  (3)  at  Snowbank  and  Caca- 
quabic  lakes;   and  (4)  in  the  vicinitj'  of  Gobbemichigamma  and  Paul  lakes. 

Contact  effect  of  the  granite. — The  sediments  metamorphosed  by  the 
Griants  Range  granite  are  well  exposed  near  milepost  92  on  the  Duluth 
and  Iron  Range  Railroad,  south  of  Tower,  find  this  place  is  readily 
accessible.  They  are  also  well  exposed  on  the  Kawishiwi  River,  near  the 
mouth  of  that  river  where  it  empties  into  Farm  Lake ;  along  the  shore  east 
thereof;  and  on  the  portage  leaving  the  ba}-  of  the  river,  in  the  SE.  \  of  sec. 
30,  T.  63  N.,  R.  10  W.,  leading  southeast  to  Clearwater  Lake.  In  places 
some  conglomerates  occur,  especially  on  the  islands  in  tlie  river  in  sec.  26, 
T.  63  N.,  R.  11  AV.,  but  the  coarse  sediments   are  very  subordinate.     The 


THE  LOWER  HURONIAN.  341 

sediments  are  now  metamorphosed  to  mica-  and  amphibole-sehists.  Occa- 
sionally garnets  have  been  produced  as  the  result  of  contact  action.  The 
metamorphosed  rocks  retain  the  sharp  banding  of  the  original  sediments, 
and  there  is  also  noticeable  at  many  places  a  rapid  altei-nation  of  bands 
of  different  grain  and  composition.  These  bands  are  composed  of  feld- 
spar and  quartz,  with  epidote,  amphibole,  and  mica.  Variation  in  the 
quantity  and  combination  of  these  minerals  causes  a  difference  in  their 
appearance.  A  study  of  almost  any  of  the  exposures  in  the  area  south  of 
the  Kawishiwi  River  in  which  these  rocks  occur  as  outlined  on  the  maps 
shows  clearly  the  cause  of  the  alteration.  They  are  penetrated  at  numer- 
ous places  by  dikes  of  granite  which  are  offshoots  from  the  great  mass 
of  Giants  Range  granite  which  lies  in  contact  with  these  schists  on  the 
south.  The  granite  dikes  become  more  numerous  as  we  go  farther  south 
nearing  this  contact,  and  at  the  south  end  of  the  portage  above  referred  to, 
on  the  ridge  just  overlooking  Clearwater  Lake,  the  granite  dikes  are  more 
numerous  and  of  larger  size  than  at  other  places.  Here,  moreover,  there  is 
an  exceedingly  good  example  of  an  eruptive  breccia.  The  breccia  consists 
of  dark-gray  and  black  schist  fragments  derived  from  these  metamorphosed  . 
sediments,  which  are  cemented  by  a  matrix  of  the  grayish  and  pink  Griants 
Range  granite.  Farther  south,  within  the  main  mass  of  the  granite,  as  on 
the  islands  in  Clearwater  Lake  and  elsewhere,  it  attains  its  normal  grain 
and  characters.  In  the  dikes  its  characters  are  likely  to  vary  as  well  as  its 
grain,  depending  upon  the  size  of  the  dikes  and  the  position  in  the  dikes 
from  which  the  specimen  is  taken.  Mountain-making  movements  may  have 
affected  and  unquestionably  did  affect  these  sediments  to  some  extent.  It 
probably  aided  in  making  them  schistose.  What  other  effects  it  may  have 
had  have  been  concealed,  however,  by  the  contact  effects  of  the  intrusive 
, granite. 

In  the  vicinity  of  Snowbank  Lake,  especially  on  the  bare  hills  south- 
west of  it,  in  sees.  10  and  17,  T.  63  N.,  R.  9  W.,  the  Knife  Lake  slates  are 
well  developed  and  are  splendidly  exposed  over  large  areas.  The  slates 
have  been  intruded  by  the  Snowbank  granite  and  from  them  have  been 
produced  mica-schists,  which  predominate,  with  subordinate  amphibole- 
sehists.  The  same  kind  of  effect  has  been  produced  by  the  intrusion  of  the 
Cacaquabic  granite  on  the  slates  nearest  it,  although,  since  the  slates 
are  not  exposed  very  near  the  granite,  the  effect  is  not  so  marked. 


342  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  microscopic  study  of  these  contact  rocks  of  the  granite  shows 
nothing'  of  especial  interest.  The  rocks  are,  as  said,  mica-  (biotite-)  and 
amphibole-schists.  The  minerals  constituting  them  are  biotite  and  some 
musco\ate,  hornblende,  actinolite,  quartz,  feldspar,  epidote,  and  garnet 
occasionally. 

Contact  effect  of  the  gahhro. — The  effect  of  the  intrusion  of  the  Snow- 
bank and  Cacaquabic  granites  on  the  suiTOunding  sediments  has  been,  as 
said,   to  produce  mica-schists,   and,   in  a  subordinate  degree,  amphibole- 
schists.     Subsequent  to  this  intrusion  these  schists  must  have  been  affected 
by  orographic  movements,  but  the  effect  of  these  movements  is  not  recog- 
nizable, for  at  a  still  later  date  the  great  Keweenawan  gabbro  was  intruded 
into  the  rocks  of  this  district  and  produced  important  contact  effects  upon 
them.     It  is  practically  impossible  now  to  discriminate  between  the  products 
of  these  different  periods  of  metamorphism.     In  general  the  sediments  in 
the  vicinity  of  the  granites  and  gabbro  have  been  transformed  into  mica- 
schists  and  amphibole-schists,  whose  chief  characters  may  have  been  pro- 
duced by  the  intrusion  of  the  Snowbank  and  Cacaquabic  granites  alone. 
South  and  southwest  of  the  Snowbank  granite,  however,  in  the  vicinity  of 
the  gabbro,  fine-grained  rocks  are  very  commonly  spotted  and  resemble  the 
so-called  spilosite,  the  spotted  contact  rocks  of  the  diabase.     This  pecuhar 
phase  of  the  metamorphism  is  probably  due  to  the  contact  action  of  the 
gabbro.     Some  of  these  spotted  rocks  are  at  present  about  thi-ee-fourths 
of  a  mile  away  from  the  nearest  exposures  of  the  gabbro.     It  is,  however, 
not  necessary  to  believe  that  the  gabbro  was  able  to  affect  the  sediments  at 
this  distance.     It  seems  almost  certain  that  at  one  time  the  gabbro  extended 
farther  north  than  the  line  where  its  present  northern  boundary  appears, 
and  that  at  that  time  it  overlay  these  rocks,  so  that  in  reality  it  was  sepa- 
rated vertically  from  the  rock  at  the  level  of  the  present  exposures  by 
perhaps   only  a  few  hundred  feet  of  intervening  rock   at  most.     Similar 
spotted  rocks  occur  in  the  slates  upon  the  prominent  hill  north  of  the  west 
end  of  Paul  Lake,  in  sec.  32,  T.  65  N.,  R.  5  W.     The  total  absence,  near 
these  spotted  sediments  in  this  area,  of  any  acid  intrusives  which  could  have 
produced  them,  and  their  presence  here  in  close  proximity  to  the  gabbro, 
seems  to  show  pretty  conclusively  that  the  spotted  rocks  in  this  place,  as 
Avell  as  the  similar  ones  mentioned  above  as  occurring  south  and  south- 
west of  the  Snowbank  granite,  are  due  to  the  gabbro  contact  action,  and 
not  to  that  of  the  granite. 


THE  LOWER  HURONIAN.  343 

Some  of  the  best  places  at  which  to  study  the  slates  that  have  been 
extremely  metamorphosed  are  on  the  east,  southeast,  and  south  shores  of 
Gobbemichigamma  Lake.  Here  the  metamorphism  of  the  slates  by  the 
gabbro  has  not  been  complicated  by  a  previous  metamorphism  of  the  slates 
by  a  granite,  as  is  the  case  in  the  vicinity  of  Snowbank  Lake.  At  various 
places  here  the  sediments  are  exposed,  showing  in  places  their  characteristic 
banding,  but  they  have  been  so  extremely  altered  that  but  for  this  banding 
their  derivation  from  the  slates  might  not  be  recognized.  The  sediments 
have  acquired  for  the  most  part  a  granular  character  and  brownish  coloi", 
and  weather  rather  readily.  As  the  result  of  their  peculiar  saccharoidal 
appearance  the  name  "muscovado,"  having  reference  to  their  resemblance  to 
brown  sugar,  was  given  to  them  b}^  Alexander  Winchell."  Their  true  char- 
acter was  not  recognized  by  him.  Since  it  is  clear  that  they  are  but  meta- 
morphosed phases  of  the  sediments,  it  seems  totally  unnecessarv  to  continue 
the  use  of  this  term,  which  can  not  be  applied  to  a  rock  of  definite  composi 
tion,  especially  since,  as  pointed  out  by  H.  V.  WinchelP  and  U.  S.  Grant," 
two  different  kinds  of  rocks,  metamorphosed  sediments,  and  certain  phases 
of  the  gabbros,  are  included  under  this  term  merely  because  they  bear  a 
superficial  resemblance  to  one  another.  This  metamorphism  has  been  pro- 
duced by  the  Duluth  gabbro,  which  at  a  number  of  places  has  been  found 
in  direct  contact  with  these  rocks.  One  of  the  best  places  at  which  to  study 
the  relationship  is  on  the  sn:iall  island  crossed  by  the  town  line  on  the 
southeast  side  of  Gobbemichigamma  Lake,  and  also  on  the  point  north  of  the 
portage  from  this  lake  east  into  Peter  Lake.  At  these  places  the  sediments 
are  overlain  by  the  gabbro,  and  the  contact  line  between  them  can  be  traced 
ver}^  clearly.  The  vertical  thickness  of  the  contact  rock  as  measured  on 
Gobbemichigamma  Lake  seems  not  to  have  exceeded  50  feet.  At  many 
places  along  the  shore  there  is  a  horizontal  exposure  of  much  more  than 
this  in  width.  This  represents,  however,  the  beveled  edge  of  the  contact 
zone,  and  since  no  data  for  the  reconstruction  of  the  removed  material 
showing  the  inclination  of  the  surface  overlain  by  the  gabbro  can  be 
obtained,  no  measurement  can  be  made  of  the  true  width  of  the  contact 
zone. 

On  the  island  referred  to  above,  and  immediately  next  to  the  gabbro, 

a  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Fifteenth  Ann.  Rept.,  1886,  pp.  183  and  351. 
6  Ibid.,  Seventeenth  Ann.  Eept.,  1888,  p.  130. 
clbid.,  Final  Eept.,  Vol.  IV,  1899,  p.  178. 


344  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  sediments  are,  as  above  stated,  granular  and  in  character  resemble  much 
more  closely  the  gabbro  than  they  do  the  sediments  proper.  At  some 
distance  away  from  the  gabbro  contact  rounded  masses  of  dense  greenish 
and  gray  rock  begin  to  appear,  surrounded  by  the  granular  contact  product. 
This  gives  a  conglomeratic  appearance  to  the  ex^josm-e.  A  little  farther 
away  from  the  gabbro,  on  a  vertical  exposure  at  the  lake  shore,  these 
pebble-like  areas  were  seen  to  be  aligned,  and  eventually  to  pass  into 
parallel  unbroken  bands.  The  explanation  of  this  occurrence  is  that  the 
gabbro  in  contact  with  the  sediments  caused  them  to  be  altered  to  the 
peculiar  granular  contact  product.  The  alteration  naturally  was  most 
eifective  nearest  the  gabbro,  and  gradually  spread,  following  along  the 
cracks  in  the  rocks,  the  vertical  as  well  as  the  horizontal  cracks.  Near  the 
gabbro  all  of  the  sediments  were  changed.  Farther  away  the  blocks  or 
fragments  of  sediments  were  changed  on  the  exterior  most  completely. 
Sometimes  this  change  was  so  far  reaching  as  to  convert  into  the  granular 
contact  product  most  of  the  sedimentarj^  block,  and  to  leave  only  a  small 
core  of  the  block,  but  even  this  was  very  much  modified.  Such  a  core 
looks  like  a  pebble  in  a  matrix,  and  gives  the  rock  a  conglomeratic  appear- 
ance. Still  farther  away  tlie  alteration  was  less,  following  only  along 
certain  of  the  most  prominent  parting  planes,  and  leaving  the  sediments  in 
bands. 

PETEOGKAPHIC    CHARACTERS    OF   THE    METAMORPHOSED    SLATES. 

Microscopic  characters. — It  would  require  a  very  much  more  detailed 
study  of  these  metamorphosed  slates  than  has  yet  been  made  to  enable  one 
to  describe  fully  the  various  changes  through  which  the  original  sediments 
have  passed  in  becoming  the  various  kinds  of  rocks  above  described,  and 
numerous  chemical  analyses  would  be  required  in  order  to  determine  just 
what,  if  any,  changes  in  chemical  composition  had  been  produced  by  the 
contact  action  of  the  gabbro. 

The  chief  constituents  of  these  contact  rocks  are:  Plagioclase  feldspar, 
little  quartz,  biotite,  muscovite,  chlorite,  green,  bluish,  and  brownish 
hornblende,  light-green  pyroxene,  hypersthene,  olivine  (?),  titanite,  epidote, 
garnet,  and  magnetite.  The  mica-,  hornblende-,  and  pyroxene-schists  and 
gneisses  derived  from  the  slates  by  the  conttict  action  of  the  gabbro  almost 
invariably  contain  very  little  (juartz,  but  are  full  of  a  rich-brown  mica 


THE  LOWER  HURONIAN.  345 

and  hornblende,  with  feldspar  present  in  large  quantity.  The  dark  con- 
stituents predominate,  and  the  mica  is  usually  more  abundant  than  the 
hornblende.  With  these  constituents  there  occur  varjang  amounts  of 
hypersthene,  light-green  pyroxene,  olivine  (I),  and  magnetite.  In  some  of 
these  gabbro  contact  rocks  the  hypersthene  exceptionally  is  the  predominant 
constituent,  usually  associated  with,  considerable  mica  and  magnetite.  In 
general  we  may  say  that  the  production  of  minerals  rich  in  magnesium  and 
iron,  in  particular  hypersthene,  brown  mica,  and  magnetite,  is  characteristic 
of  the  gabbro  contact.  These  are,  of  course,  the  kinds  of  minerals  whi^ch,  a 
priori,  would  be  expected  in  rocks  greatly  affected  by  a  gabbro  magma. 
Analyses  of  these  rocks  and  of  the  rocks  from  which  they  were  derived 
have  not  been  obtained,  and  indeed  a  great  many  would  be  required  to 
prove  the  thesis  that  an  actual  transfer  of  material  from  the  gabbro  to  the 
surrounding  sediments  had  taken  place.  However,  in  view  of  the  produc- 
tion in  such  large  quantity  of  the  magnesian  and  iron  minerals  in  these 
sediments  it  is  believed  that  such  a  transfer  of  some  magnesia  and  iron  has 
actually  taken  place  from  the  gabbro  magma  to  the  sediments  now  con- 
taining these  minerals.  The  fact  that  these  minerals  occur  more  abun- 
dantly in  the  rocks  near  the  gabbro  than  in  those  farther  away  supports 
this  view. 

The  spotted  contact  rocks,  the  spilosites,  are  known  to  occur  as  the 
result  of  the  contact  action  of  diabases  and  gabbros,  and  those  occui-ring 
in  this  district  are  believed  to  be  without  doubt  the  product  of  the  gabbro 
contact  and  to  be  characteristic  of  it.  The  spilosites  from  -the  Vermilion 
district  are  fairly  common  in  the  area  southwest  of  Snowbank  Lake.  They 
are  very  similar  to  the  spilosite  described  from  the  Crystal  Falls  district  of 
Michigan,"  and,  like  them,  the  spots,  wliich  are  in  general  of  oval  outline, 
occur  isolated  or  united  along  the  long  axis  of  the  ovals  in  a  series.  These 
spots  are  composed  of  aggregates  of  muscovite,  epidote,  little  chlorite,  and 
sphene,  in  a  tine  groundmass  (which  predominates)  of  muscovite,  chlorite, 
epidote,  sphene,  feldspar,  and  quartz. 

Somewhat  different  are  the  spotted  rocks  occurring  north  of  Paul  Lake, 
for  example.  The  white  material  forming  the  spots  has  essentially  the  same 
single  and  double  refraction  as  feldspar.     The  material  includes  biotite  and 

a  Mon.  U.  S.  Geol.  Survey  Vol.  XXXVI,  1899,  pp.  206-207.     Also  a  contribution  to  the  study  of 
contact  metamorphism,  by  J.  Morgan  Clements:  Am.  Jour.  Sci.,  4th  series,  Vol.  VII,  1899,  pp.  81-91. 


346  THE  VERMILION  IRON-BEARING  DISTRICT. 

chlorite  flakes,  and  particles  of  iron  ore.  Grant"  has  described  cordierite 
occurring  in  similar  rocks  on  Gobbemichigamma  Lake,  to  the  southwest 
of  the  area  above  referred  to.  Very  possibly  the  material  forming-  these 
white  spots  is  cordierite,  but  no  conclusive  proofs  of  this  were  obtained. 


THICKNESS. 


It  has  already  been  stated  that  in  portions  of  tlie  Vermilion  district 
the  Knife  Lake  slates  show  a  very  great  width.  From  this  fact  alone  one 
not  taking  into  consideration  the  intensely  plicated  condition  of  these  rocks 
and  hence  the  possibilit}^,  or  even  the  certainty,  of  more  or  less  repetition, 
niig:ht  be  led  to  infer  that  these  slates  are  of  enonnous  thickness. 

This  folding,  of  course,  points  to  a  jirobable  reduplication  of  the  beds. 
While  the  slate  area  can  by  no  means  be  described  as  homogeneoiis, 
nevertheless  it  is  true  that  clearly  recognizable  key  rocks  are  wanting. 
Consequently  there  must  be  numbers  of  anticlines  and  S3"nclines  which  it 
has  not  been  possible  under  the  existing  conditions  to  ,recognize.  These 
facts  render  it  exceedingly  difficult  to  make  any  authoritative  statement  as 
to  the  thickness  of  tliis  series.  At  one  place,  however,  the  structm-e  is 
fairly  simple.  Between  Ogishke  Muncie  Lake  and  Gobbemichigamma 
Lake,  two  of  the  large  lakes  in  the  eastern  part  of  the  district,  there  are 
two  small  lakes,  Fox  and  Agamok  lakes,  which  occupy  the  low  ground. 
North  of  these  there  is  a  high  ridge  occupied  by  Archean  greenstone  with 
the  Ogishke  conglomerate  on  its  southern  flank.  South  of  this  string  of 
lakes  there  is  a  second  ridge  of  Archean  greenstone,  forming  a  verv  marked 
topographic  feature.  Between  these  two  ridges  we  find  the  Knife  Lake 
slates,  showing  a  number  of  good  exposures.  Traverses  across  this  area 
from  north  to  south  gave  at  first  a  series  of  south  dips,  gradually  becoming 
flatter  and  flatter,  until  the  bottom  of  the  syncline  just  south  of  the  lakes 
mentioned  as  lying  in  the  bottom  of  the  depression  was  reached,  where  the 
flattest  dips  were  found.  Continuing  across  as  we  ascend  the  south  ridge, 
this  succession  of  dips  is  repeated  in  reverse  order — i.  e.,  the  steepest  dips  are 
nearest  the  ridge,  and  the  dips  are  to  the  north.  Moreover,  it  was  possible 
to  observe  a  repetition  of  the  rocks.  The  slates  hei-e  evidently  occupied  a 
distinct  syncline,  and  moreover  this  syncline  seems  to  be  a  simple  one. 

"Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  pp.  657-658. 


THE  LOWER  HURONIAN.  347 

From  the  data  obtained  here  the  total  thickness  of  the  slates  across  this  syii- 
cline  is  calculated  to  be  5,000  feet,  which  divided  in  half  gives  us  a  maxi- 
mum thickness  here  of  2,500  feet.  It  is  impossible  to  state  positively 
whether  or  not  this  represents  the  maximum  thickness  of  the  slates  for  this 
area.  The  presumption  is,  however,  that  the  maximum  thickness  here 
obtained  does  not  represent  the  maximum  original  thickness  of  the  slates, 
as  they  must  have  originally  occupied  the  entire  basin  between  the  older 
rocks,  and  possibly  had  a  much  greater  vertical  extent,  the  rocks  above 
those  now  remaining  having'  been  of  course  long  since  removed  by  erosion. 

INTERESTING   LOCALITIES. 

Structure  and  relations  of  the  slates. — On  Carp  and  Birch  lakes  the 
relations  of  the  topography  to  the  geologic  structure  are  well  indicated. 
The  central  headland  of  Carp  Lake,  as  well  as  much  of  the  western  bay 
and  the  headlands  beside  the  narrows,  are  composed  largely*  of  a  coarse 
graywacke  which  is  characteristic  of  the  lower  part  of  the  slate  formation. 
These  headlands  are  directly  along  the  strike  of  the  plunging  anticline  at 
the  east  end  of  the  lake.  Corresponding  to  them  there  is  an  eastward- 
projecting  headland  in  the  west  bay  of  the  lake,  and  this  also  is  composed 
of  the  coarse  graywacke.  The  southern  part  of  the  western  bay  is  com- 
posed of  the  normal  slates.  Thus,  while  Carp  Lake  is  as  a  whole  a  part 
of  a  monocline  dipping  to  the  south,  it  very  plainly  has  a  subordinate 
anticline  through  the  center  which  makes  the  graywacke  at  the  base  of  the 
slates  "the  predominant  rock  exposed. 

Birch  Lake,  west  of  Carp  Lake,  was  found  to  be  surrounded  by  slates 
for  about  a  mile  at  its  eastern  end.  Then,  at  a  little  bay  occurring  just 
east  of  the  long  westward-projecting  point,  greenstone  comes  out  upon  the 
north  shore  for  a  short  distance.  The  slates  appear  again  upon  the  point 
to  the  southwest  of  the  greenstone.  The  greenstone  is  again  exposed  in 
the  next  bay  to  the  north,  and  with  a  minor  fold  swings  up  into  the 
extreme  north  bay  and  thence  inland,  the  shore  being  composed  of  slate. 
The  relations  are  such  as  to  indicate  a  series  of  infolds,  the  slates  occupying 
the  depressions,  appearing  as  reentrants  on  the  lakes,  and  the  greenstones 
occupying  the  headlands  and  highlands  to  the  north. 

Topographic  depressions  usually  separate  the  Archean  greenstones 
from  the  slates  which  lie  near  them,  hence  the  sequence  of  rocks  from  the 


348  THE  VERMILION  IRON-BEARING  DISTRICT. 

greenstones  to  the  slates  is  usually  interrupted.  A  number  of  contacts 
have  been  found,  however,  and  in  every  instance  where  careful  examination 
was  made  at  least  a  narrow  belt  of  conglomerate  derived  from  the  green- 
stone intervened  between  the  greenstone  and  the  slates.  The  localities  at 
which  such  conglomerates  occur  are  described  under  the  heading  "Ogishke 
conglomerate"  (p.  317  et  seq.).  These  conglomerates  were  found  to  be  pres- 
ent in  practically  every  case  in  which  careful  search  was  made,  and  this  fact 
is  the  warrant  for  indicating  on  the  maps  in  many  places  continuous  belts  of 
conglomerate  between  the  Knife  Lake  slates  and  the  Ely  greenstones.  This 
insertion  has  been  made  especially  for  the  reason  that  thereby  the  structm-e 
of  the  district  can  be  made  plainer.  In  almost  every  case,  however,  in 
order  to  show  this  belt,  it  has  been  necessary  to  magnify  many-fold  the 
true  thickness  of  the  conglomerate.  In  the  NW.  \  of  sec.  17,  T.  63  N.,  R.  9 
W.,  contacts  between  the  Knife  Lake  slates  and  the  ellipsoidal  Ely  green- 
stone can  be  found  at  many  places.  Here  there  is  an  eastward-plunging 
trough  of  sediments  infolded  in  the  greenstones.  Here  and  there  a  narrow 
band  of  conglomerate  occurs  between  the  slates  and  the  greenstone;  else- 
where, however,  actual  contacts  have  been  found  in  which  the  slate  lies 
next  to  a  zone  of  very  schistose  greenstone,  which  then  grades  down  into 
the  normal  greenstone.  Evidently  these  deposits  were  formed  in  a  fairly 
protected  area,  as  is  indicated  by  the  predominance  of  the  slates  over  the 
coarser  gray  wackes  and  the  scarcity  of  conglomerates  along  the  border  of 
the  greenstone. 

About  120  paces  south  of  the  Moose  Lake  end  of  the  Moose  Lake- 
Flask  Lake  portage  there  occur  numerous  small  outcrops  of  a  porphyritic 
greenstone  which  has  a  more  or  less  schistose  green  matrix,  with  numerous 
white  porphyritic  crystals  of  feldspar  evenly  distributed  through  it. 
Toward  Moose  Lake  this  porphyry  becomes  more  and  more  fissile  until 
finally  it  passes  into  a  schist.  This  schist  is  so  beautifully  and  finely 
laminated  and  fissile  that  it  can  almost  be  spoken  of  as  a  slate.  Upon  tlie 
surfaces  the  flattened  phenocrysts  are  shown.  This  schist  belongs  with  the 
Knife  Lake  slates.  South  of  it  lies  a  graywacke  made  up  of  disintegrated 
material  derived  from  the  porphyry,  with  which  it  lies  directly  in  contact. 
There  are  no  pebbles  in  this  material  which  can  be  clearly  recognized  as 
such.  If  any  exist  they  merge  into  the  matrix,  and  the  two  rocks,  the 
schistose  porphyry  and  the  grit  derived  from  it,  resemble  ench  other  so 


THE  LOWER  HURONIAN.  349 

strongly    that    it    is    onl}^    after    very    close    study    that    they    can    be 
discriminated. 

At  the  southwest  end  of  the  lake,  whose  tip  just  extends  into  the  NE. 
J  of  sec.  11,  T.  64  N.,  R.  8  W.,  there  is  a  dike  of  granite-porphyry  which 
cuts  across  the  bedding.  Other  granite  dikes  must  occur  throug-hout  this 
district  cutting  these  slates,  although  few  of  them  have  been  found.  One, 
for  instance,  occurs  upon  the  south  side  of  the  large  island  in  Knife  Lake 
north  of  sec.  31,  T.  65  N.,  R.  8  W. 

Contact  metamorpJdsm  of  the  slates. — An  excellent  place  at  which  to  see 
the  character  of  the  metamorjjhosed  Knife  Lake  slates  is  along  the  Duluth 
and  Iron  Range  Railroad,  near  milepost  92.  This  place  is  also  very  easily 
reached. 

In  going  south  from  Tower  after  passing  the  south  side  of  the  Ely 
greenstone  one  observes  the  following  series  of  sediments: 

On  the  fii'st  exposure  south  of  the  greenstone  we  get  slates  and  gray- 
wackes,  which  are,  even  in  this  first  exposui'e,  somewhat  metamorphosed — 
that  is,  they  are  very  firm  slates  and  quite  thoroughly  indurated  gray  wackes, 
but  still  show  verj  clearl}^  their  unmistakable  sedimentary  characters.  The 
first  gi-anite  dikes  observed  in  these  sediments  occur  about  a  mile  south  of 
their  most  northern  outcrop.  Continuing-  south  from  the  first  outcrops  of 
the  sediments  on  the  railroad,  the  ones  to  which  we  come  thereafter  take 
on  a  more  and  more  altered  character  as  we  go  farther  south.  At  milepost 
92  the  presence  of  good  conglomerates  with  accompanying  finer  sediments 
showing  well-marked  sedimentary  structures  such  as  normal  and  current  bed- 
ding, indicates  with  absolute  clearness  the  mode  of  origin  of  the  rocks.  The 
microscopic  study  of  these  rocks  is  not  so  satisfactory  as  the  macroscopic; 
the  changes  which  the  rocks  have  undergone  have  obliterated  all  features 
which  would  enable  one  to  determine  with  absolute  accuracy  by  means  of 
the  microscope  the  original  characters  of  these  rocks,  although  the  coarse 
macroscopic  structures  still  remain.  Farther  south  the  outcrops  become 
scarcer  as  we  approach  the  great  muskeg  area  in  the  low  ground  north  of 
the  Giants  range.  Nevertheless  here  the  few  outcrops  which  were  observed 
are  exceedingly  indurated  banded  rocks  which  can  be  more  properly  spoken 
of  as  mica-schists  than  as  graywackes.  Still  farther  south  the  rocks  are 
mica-schists  and  mica-gneisses,  with  very  much  contorted  banding,  and 
.  are  cut  by  granite  dikes.     The  change  in  the  character  of  the  rocks  as 


350  THE  VERMILION  IRON-BEARING  DISTRICT. 

shown  on  the  outcrops  seems  to  point  conclusively  to  the  fact  that  these 
changes  are  due  to  an  increasing  metamorphism  of  these  sediments,  con-e- 
sponding  to  our  approach  to  the  main  Giants  Range  granite  mass  lying  far 
to  the  south.  In  this  western  portion  of  the  area  the  center  of  intrusion 
lies  in  the  Giants  range,  a  number  of  miles  south  of  the  area  descril^ed  in 
this  monograph. 

The  metamorphism  of  these  Knife  Lake  slates  can  be  seen  to  advantage 
in  the  vicinity  of  Snowbank  Lake.  On  the  north  shore  of  Snowbank  Lake, 
at  a  number  of  places,  mica-schists  may  be  found  wliich  are  cut  through  and 
thi'ough  by  dikes  derived  from  the  Snowbank  granite.  These  schists,  when 
followed  inland  for  a  considerable  distance,  are  found  to  grade  into  the  less 
and  less  altered  sediments,  until  eventually  the  normal  Knife  Lake  sedi- 
ments are  reached.  Somewhat  similar  metamorphosed  mica-schists  can  be 
observed  upon  the  portage  between  Round  Lake  and  Disappointment  Lake. 
These  have  been  metamorphosed  by  the  gabbro  which  lies  only  a  short 
distance  to  the  south,  although  in  all  probability  they  were  first  affected  by 
the  intrusion  of  the  Snowbank  granite.  Here  'the  cleavage  is  parallel  to 
the  bedding.  This  mica-schist  resembles  in  a  remarkable  degree  the  mica- 
schist  "  which  occurs  as  the  upper  part  of  the  Upper  Slate  division  of  the 
sediments  at  English  Lake,  near  Penokee  Gap.  The  most  crystalline  part 
of  this  Upper  Slate  member  runs  from  Penokee  Gap  westward,  and  here  the 
basal  member  of  the  Keweenawan,  lyixig  north  of  it,  is  a  g'reat  mass  of 
gabbro.  The  lower  members  of  the  Upper  Slate  at  Penokee  Gap,  although 
at  lower  horizons,  and  therefore  presumably  more  deeply  buried,  and 
moreover  containing  unquestionable  intrusions  of  the  diabase,  are  never- 
theless composed  of  comparatively  little  metamorphosed  black  slate.  It 
seems  conclusive  that  these  mica-schists  in  the  vicinity  of  Snowbank  Lake 
and  those  occurring  in  Penokee  Gap  both  owe  their  present  characters  to 
the  alteration  of  an  original  slate  by  the  gabbro. 

At  a  point  only  a  few  miles  west  of  ^Montreal  River,  and  again  at  the 
top  of  the  Upper  Slate  member,  near  tlie  gabbro,  the  rock  is  a  gra}^,  coarse, 
strongly  micaceous  graywacke,  the  only  recognizable  clastic  material  in  the 
rock  being  the  coarse  quartz  and  feldspar.  In  Michigan,  on  the  Penokee- 
Gogebic  range,  east  of  ]\Iontreal  River,  the  bottom  members  of  the 
Keweenawan  are    comparatively  thin-bedded   lava    flows,   dolerites,   and 


«Mon.  U.  S.  Geol.  Survey  Vol.  XIX,  1892,  pp.  302-308. 


THE  LOWER  HURONIAN.  351 

amygdaloidal  lavas,  and  here  the  top  layers  of  the  Upper  Slate  are  the 
ordinary  or  slightly  metamorphosed  black  slates.  From  this  it  appears 
clear  that  on  the  Penokee  rang-e  the  metamorphism  of  the  slates  and  mica- 
schists  is  due  to  the  gabbro,  and  the  same  conclusion  seems  to  follow  for 
the  similar  schists  in  the  Vermilion  district. 

The  Lower  Huronian  slates  appear  at  a  number  of  places  on  the 
north,  south,  and  southeast  shores  of  Gobbemichigamma  Lake.  On  the 
southeast  and  east  shores  these  slates  are  in  many  places  found  in  direct 
contact  with  the  Keweenawan  gabbro,  and  where  near  it  or  in  contact  with 
it  they  have  been  extremely  metamorphosed.  Much  less  metamorphism  is 
noticeable  on  the  slates  occurring  on  the  north  side  of  the  lake.  That  part 
of  the  south  shore  of  the  lake  in  the  immediate  vicinity  of  the  section 
line  between  sec.  1,  T.  64  N.,  R.  6  W.,  and  sec.  6,  T.  64  N.,  R.  5  W.,  affords 
a  good  opportunity  for  studying  the  relations  between  these  two  rocks. 
There  is  a  high  cliff  at  the  point  where  the  section  line  above  mentioned 
comes  to  the  shore.  On  this  cliff  one  can  readily  detect  bands  of  rock 
which  strike  N.  10°  E.  magnetic  and  dip  20°  to  the  east.  This  strike  as 
taken  really  represents  the  strike  of  the  face  of  the  cliff.  Were  it  possible 
to  obtain  it,  the  true  strike  of  the  beds  would  be  found  to  be  somewhat  dif- 
ferent. The  bands  in  the  cliff  are  made  up  of  very  dense-grained,  hard, 
siliceous  rock.  They  are  rarely  more  than  4  inches  in  thickness  and  fre- 
quently very  much  thinner.  Alternating  with  these  bands  there  is  a 
rotten,  brown,  coarse-grained  material  which  seems  to  weather  very 
readily — it  certainly  does  so  in  comparison  with  the  adjacent  harder 
bands — and  appears  much  like  gabbro.  If  we  follow  the  shore  around  to 
the  northeast  it  is  there  possible  to  land  and  ascend  the  sloping  hill  leading 
to  the  top  of  the  above-mentioned  cliff.  In  the  ascent  of  this  hill  one 
crosses  the  bands  of  rock,  which  are  imperfectly  shown  in  this  section. 
The  harder  bands  are  especially  recognizable,  as  they  are  likely  to  form 
shelves,  the  slopes  between  being  formed  of  the  softer  gabbro-like  material. 
The  relations  here  seem  to  indicate  either  that  the  banded  sedimentary  has 
been  included  in  the  gabbro  or  else  that  the  gabbro  has  been  thoroughly 
injected  into  the  sedimentar}^,  the  injection  following  chiefly  the  bedding 
planes  as  planes  of  least  resistance.  The  normal  coarse-grained  gabbro 
occurs  on  the  shore  just  a  short  distance — about  a  quarter  of  a  mile — back 
of  this  headland.     Following  this  shore  from  the  cliff  to  the  west  we  note 


352  THE  VERMILION  IRON-BEARING  DISTRICT. 

ai^pearing  at  the  water  level  a  couglomeratic  looking  rock,  the  pebbles  of 
which  seem  to  be  a  dense  quartzitic  graywacke  or  slate,  whereas  the 
matrix  is  light  colored,  rather  coarse  grained,  and  appears  like  an  exceed- 
ingly feldspathic  biotite-gabbro.  This  conglomeratic  rock  is  cut  by  a  sheet 
of  basalt  12  to  16  inches  wide,  which  is  very  nearly  horizontal,  showing- 
only  very  slight  east-west  rolls.  The  basalt  sheet  is  separated  into 
very  symmetrical  hexagonal  columns,  and  shows  a  distinct  fine  basalt 
selvage,  while  at  its  center  the  sheet  is  coarser  grained.  Ascending  the  cliff, 
above  tliis  conglomeratic  rock,  we  pass  from  it  into  a  coarse  brown,  more 
or  less  friable  rock,  which  calls  to  mind  Winchell's  muscovado.  Still  higher 
up  this  rock  is  found  to  grade  vertically  into  a  coarse  normal  gabbro.  This 
conglomeratic-looking  rock  continues  along  the  shore  still  farther  to  the 
southwest. 

Following  the  shore  of  Gobbemichigamma  Lake  to  the  east  of  our 
starting  point  at  the  cliff,  a  pseudo-conglomeratic  rock  similar  to  that 
described  above  begins  on  the  shore  just  a  little  north  of  the  meander 
corner  of  the  town  line  between  Ts.  64  and  65  N.,  R.  5  W.  The  rock  here 
is  rather  fine-grained  granular  rock,  weathering  white  to  yellow  and  brown, 
in  which  occur  very  frequent  rounded  areas  all  essentially  alike  and 
seemingly  of  one  kind  of  rock — a  dense,  green,  fine-grained  very  quartzose 
graywacke.  A  similar  rock  is  well  exposed  on  the  little  island  just  west 
of  the  shore  on  which  the  meander  corner  stands,  and  here  also  its  relations 
to  the  gabbro  and  its  true  characters  are  better  shown  than  elsewhere. 
The  occurrence  observed  here  has  already  been  described  (p.  343).  The 
gabbro  is  evidently  younger  than  the  pseudo-conglomeratic  rock,  which 
has,  in  fact,  been  produced  from  preexisting  sedimentary  rocks  by  the 
intrusion  into  them  and  extensive  metamorphism  of  them  by  the  gabbro. 

SSCTION  11— ACID  AND  BASIC  INTRUSIVES  OF  THE  LOWER  HURONIAN. 

I]SrTKODUCTIO>r. 

In  Section  III  of  Chapter  III  various  acid  intrusives  which  are  of  the 
same  general  petrographic  character  and  geologic  age  are  discussed.  In 
addition  to  these  intrusives,  there  are  found  in  the  Vermilion  district 
thi-ee  other  large  masses  of  granite  and  granite-porpln-ry,  from  which 
iiumerous  dikes  have  been  given  off.    These  large  masses  and  accompanying 


THE  LOWER  HURONIAN.  353 

dikes  penetrate  the  siuToundiug-  Lower  Huronian  sediments  and  other  adja- 
cent rocks.  The  large  masses  especially  have  produced  on  the  adjacent 
rocks  far-reaching  metamorphism.  In  addition  to  these  main  areas  and 
the  dikes  which  can  be  connected  with  them,  there  are  throuo-hont  the 
district  acid  dikes  which  vary  somewhat  in  petrographic  characters  and 
which  penetrate  the  various  formations  adjacent  to  them.  The  largest 
masses  of  these  acid  rocks  occur  in  the  core  of  the  Giants  range,  in  the 
vicinity  of  White  Iron  Lake,  extending  northeast  and  southwest  of  that 
area,  and  in  the  vicinity  of  Snowbank  and  Cacaquabic  lakes,  and  are 
described  below.  In  addition,  a  section  will  be  devoted  to  a  brief  description 
of  the  various  dikes  which  can  not  properly  be  connected  with  these 
masses,  but  are  assumed  to  be  of  essentially  the  same  age.  Still  another 
section  is  given  to  a  very  brief  description  of  certain  basic  and  intermediate 
dikes  which  have  been  found  to  bear  the  same  relations  to  the  adjacent 
formations  as  the  acid  intrusives  bear,  and  which  are  hence  presumed  to  be 
of  the  same  age.  '  ' 

GIAISTTS  RAJiTGE    GRAISTITE. 

DISTRIBUTION,   EXPOSURES,   AND   TOPOGRAPHY. 

Distribution. — The  Giants  Range  granite  borders  a  portion  of  the  south- 
ern side  of  the  Vermilion  district,  and  is  best  developed  in  that  part  of  the 
area  extending  from  the  vicinity  of  Beaver  River,  in  sees.  31  and  32,  T.  62  N., 
R  13  W.,  eastward  into  sees.  24  and  25,  T.  63  N.,  R.  10  W.  Immediately  in 
the  vicinity  of  White  Iron  Lake  it  reaches  a  very  extensive  development, 
and  to  the  southwest,  in  the  portion  of  the  Giants  range  in  the  Mesabi 
district,  it  occupies  the  core  of  the  range,  and  here  shows  its  typical 
characters,  and  has  therefore  been  called  the  Giants  Range  granite.  The 
granite  underlies  a  very  miich  greater  area  than  is  shown  on  the  map 
(PI.  II).  According  to  the  Minnesota  maps,  and  also  as  observed  in  a 
reconnaissance  trip,  it  extends  a  number  of  miles  to  the  south  of  the  area 
here  described,  where  it  is  bordered  by  the  Duluth  gabbro  mass.  This 
gabbro  mass  likewise  cuts  across  it  in  the  northeast-southwest  direction, 
gradually  nearing  the  area  underlain  by  the  granite  as  it  is  followed  east- 
ward, until  in  sec.  19,  T.  63  N.,  R.  9  W.,  the  granite  is  completely  cut  out 
by  the  gabbro.  The  area  underlain  by  this  granite  thus  varies  from  a 
very  narrow  strip  on  the  east,  where  it  actually  feathers  out,  to  an  area  5 
MON  XLV — 03 23 


354  THE  VERMILION  IRON-BEARING  DISTRICT. 

or  6  miles  across,  at  a  point  south  of  White  Iron  Lake.  The  portion  in 
the  area  described  in  this  report  is  rarely  more  than  2  miles  wide  and 
usually  less  than  that.  Indeed,  that  part  of  the  granite  -s^-liich  was  actually 
studied  represents  merely  the  border  of  the  granite,  for  the  reason  that  in 
most  cases  our  traverses  were  ended  at  the  northern  border,  as  the  prime 
object  of  the  survey  was  the  delimitation  and  study  of  the  formations  in 
the  iron-bearing  district  proper. 

Exposures. — The  exposures  of  granite  are  good,  as  the  area  in  which 
it  lies  is  well  dissected  by  streams  and  contains  a  number  of  lakes.  The 
exposures  are  especially  good  on  White  Iron  Lake,  in  T.  62  N.,  Rs.  11 
and  12  W. 

Topography. — The  topography  is  that  usually  seen  in  the  granitic  areas 
of  the  Lake  Superior  region,  consisting  of  low  rounded  to  oval  hills  with 
lakes  here  and  there,  in  the  intervening  valleys.  The  range  of  hills  formed 
by  the  Giants  Range  granite  is  the  topographic  continuation  to  the  north- 
east of  the  Mesabi  or  Griants  range  of  the  Mesabi  district.  The  hills  are 
nowhere  high,  however,  and  do  not  show  very  well  the  character  of  a  hill 
range. 

About  2  miles  southeast  of  Ely  and  extending  from  sec.  11,  T.  62  N., 
R.  12  W.,  to  sec.  25,  T.  62  N.,  R.  13  W.,  there  is  an  area  underlain  by  this 
granite  which  is  \qyj  similar  to  that  occurring  north  of  the  district  on  Iron 
Lake  (see  p.  259).  This  area  is  almost  base-leveled,  the  lakes  into  which 
it  is  drained  representing  the  level  to  which  the  surrounding  land  has  been 
very  nearly  reduced.  This  area  is  very  much  larger  than  that  occurring 
near  the  international  boundary  before  referred  to.  Here  the  swamps  are 
extensive  and  the  elevations  are  very  slight,  being  flat  hillocks  of  granite 
rising  as  a  rule  only  a  few  feet  above  the  adjacent  low  ground. 

PETROGRAPHIC  CHARACTERS. 

Macroscopic  characters. — The  Giants  Range  granite  includes  a  series  of 
granites  ranging  in  color  from  light  gray  to  very  dark  gray,  to  flesh  color, 
pinlv,  and  red.  The  grain  varies  also  very  materially,  the  rock  passing  from 
very  dense  fine-grained  granites  through  medium  to  coarse-grained  ones. 
While  this  rock  is,  as  a  rule,  granitic  in  texture,  there  are  also  variations 
to  granite-porphyries  and  exceptionally  to  some  that  can  be  spoken  of  as 
rhyolite-porphyries.     These  granite-  and  rhyolite-porphyry  dikes  are  nor- 


THE  LOWER  HURONIAN.  355 

mally  found  cutting  the  greenstone  which  borders  tlie  Giants  Range  granite 
on  the  north.  They  are  described  with  the  Giants  Range  granite,  since  they 
are  presumed  to  be  offshoots  from  it,  although  their  direct  field  connection 
with  it  can  not  be.  shown.  Their  petrographic  similarity  to  the  main 
granite  mass  seems  alone  sufficient  to  warrant  their  description  together 
and  to  support  the  view  that  they  were  derived  from  the  same  deep-seated 
source. 

In  places  along  the  Kawishiwi  River  the  granite  is  slightly  schistose. 
This  schistosity  is  especially  noticeable  along  the  margin  of  the  granite, 
where  it  lies  next  to  the  Archean  Ely  greenstone.  These  schistose  phases 
can  be  well  seen  in  the  southern  half  of  sec.  24,  T.  63  N.,  R.  10  W.,  imme- 
diately north  of  the  Kawishiwi  River. 

The  granite  massive  includes  areas  of  dark  hornblende  and  mica  rocks 
which  are  more  or  less  schistose  and  consist  of  hornblende,  mica,  quartz, 
and  feldspar  in  about  equal  proportion.  The  relationship  which  the  granite 
is  presumed  to  have  to  these  is  indicated  by  the  use  above  of  the  word 
"includes,"  for  it  surrounds  these  masses  in  some  cases  and  sends  offshoots 
into  them.  In  other  cases  the  granite  is  found  cementing  an  eruptive 
breccia  the  fragments  of  which  were  derived  from  the  above-mentioned 
schists.  Such  a  breccia,  for  example,  is  well  shown  just  north  of  Clear- 
water Lake,  alongside  the  portage  entering  that  lake  from  the  Kawishiwi 
River.  These  fragments  in  the  granite  are  in  some  cases  derived  from  the 
Ely  greenstone.  In  other  cases  the  fragments  represent  a  sedimentary  series, 
the  Lower  Huronian,  which  has  been  intruded  and  included  by  the  granite. 
It  is  difficult  to  determine  the  original  characters  of  these  included  rock 
fragments  from  a  microscopic  study  after  they  have  been  metamorphosed. 
In  the  field  one  has  as  a  guide  the  proximity  of  the  granite  to  larger  masses 
of  metamorphosed  sediments  on  the  one  hand  or  to  the  Ely  greenstone 
on  the  other.  Naturall}^  the  fragments  in  the  granite  are  most  likely  to 
have  been  derived  from  that  rock  to  which  the  fragments  are  nearest. 

Dikes  of  very  fine-grained  red  aplite  cut  the  Giants  Range  granite. 

The  constituents  of  these  granitic  rocks  as  disclosed  by  the  microscope 
are  orthoclase .  (microcline),  plagioclase,  quartz,  hornblende,  mica,  zircon, 
apatite,  sphene,  a  little  iron  ore.  These  minerals  possess  their  usual  char- 
acters. It  is  interesting  to  note  that  the  microcline  is  especially  abundant 
in  granites  which  show  the  pressure  effects  in  the  other  minerals  more  evi- 


356  THE  VERMILION  IRON- BEARING  DISTRICT. 

(k'litly  tlian  the  rocks  with  a  smaller  amount  of  the  microclme.  This  is 
evidence  iu  favor  of  the  microcline  twinning  having  been  produced  by 
pressure.  The  quartz  and  feldspar  occasionally  are  in  niicropegraatitic 
interg'rowths.  Some  secondary  minerals  occur  with  these  rocks,  sucii  as 
chlorite,  sericite,  and  epidote. 

RELATIONS   TO   ADJACENT  FORMATIONS. 

The  Giants  Range  granite  is  found  at  different  places  in  juxtaposition 
with  the  Ely  greenstone,  the  Soudan  formation,  the  Lower  Huronian  sedi- 
ments, and  the  Keweenawan  gabbro,  enumerated  in  order  from  the  base  up. 
Its  relations  to  these  formations  are  in  each  ciase  quite  clearly  shown  and 
will  be  specifically  described  in  the  following  paragraphs. 

Relations  to  tlw  Ely  greenstone. — The  granite  cuts  the  greenstones  con- 
stituting this  formation  in  innumerable  dikes  which  individually  seem  to 
have  little  effect  upon  the  greenstone,  judging  from  the  lack  of  well-marked 
contact  zones  adjacent  to  the  dikes.  As  we  get  nearer  the  contact  between 
the  greenstone  and  the  granite  massive,  however,  exactl}-  the  same  kind  of 
metamorphic  products  are  observed  as  are  found  associated  with  the  contact 
of  the  intrusive  granite  of  Trout,  Burntside,  and  Basswood  lakes  with  the 
Ely  greenstone  on  the  northern  side  of  the  district.  As  the  dikes  increase 
in  number  the  greenstones  are  altered  to  amphibolitic  and  micaceous  schists, 
frequently  still  retaining  the  unmistakable  amygdules  and  ellipsoidal  parting 
of  the  original  greenstones.  Similar  products  of  the  granite  intrusion  have 
been  described  under  the  description  of  the  effect  of  the  granite  of  Trout, 
Burntside,  and  Basswood  lakes  on  the  Ely  greenstone  (p.  156  et  seq.). 

Relations  to  the  Soudan  formation. — The  Soudan  formation  is,  as  has  been 
stated,  of  very  limited  extent,  and  consequently  there  are  few  opportunities 
for  observing  granite  dikes  in  it.  However,  such  dikes  have  been  observed 
at  a  number  of  places  (see  p.  359),  and  their  presence  shows  clearly  that 
the  Giants  Range  granite  cuts  the  Soudan  formation  and  is  hence  younger 
than  it. 

Relations  to  the  Lower  Huronian  sediments. — The  Duluth  and  Iron 
Range  Railroad,  south  of  Tower,  especially  in  the  vicinity  of  milepost  92, 
gives  very  good  sections  through  the  Lower  Huronian  sediments  and  shows 
them  to  be  cut  by  dikes  of  granite.  These  dikes  ^■ary  in  size  on  different 
ex])osures,   ranging  from    1    incli  up   to   4  feet  in  width.     Tliey  can  not  be 


THE  LOWER  HURONIAN.  357 

connected  in  the  field  directly  with  the  granite  dikes  which  cut  the  gray- 
Embarrass  granite,"  lying  a  number  of  miles  to  the  south,  but  their  general 
characters  are  so  similar  that  they  are  presumed  to  be  of  the  same  ag-e  as 
these  dikes,  and  to  have  been  derived  from  the  same  source.  Moreover, 
farther  east  jjractically  the  same  granite  is  found  in  dikes  which  are  clearly 
offshoots  from  the  Giants  Range  granite,  and  those  at  the  extreme  western 
side  of  the  district  are  likewise  supposed  to  be  offshoots  connected  under- 
groiind  with  the  Giants  Range  granite  mass,  although  at  the  surface  they 
are  a  good  many  miles  distant  from  it.  The  metamorphosing  effect  of  the 
Giants  Range  granite  on  these  sediments  has  been  described  under  the 
description  of  the  sediments  themselves  (pp.  340-341). 

As  we  go  northeast  in  the  district  beyond  White  Iron  Lake,  especially 
along  the  Kawishiwi  River,  we  find  a  comparatively  small  area  of  sediments, 
conglomerates,  graywackes,  and  associated  slates  extending  from  near  the 
mouth  of  the  Kawishiwi  River  on  Farm  Lake,  sec.  34,  T.  63  N.,  R.  11  W., 
eastward  into  sec.  29,  T.  63  N.,  R  10  W.  The  area  underlain  by  these 
sediments  has  a  north-south  extent  from  the  Kawishiwi  River  to  Clear- 
water Lake  of  about  1^  miles.  These  rocks  are  identical  petrographically 
with  the  rocks  which  have  been  classed  with  the  Lower  Hurouian  sediments 
and  are  presumed  to  be  of  the  same  age.  They  are  cut  through  and 
through  by  dikes  of  the  Giants  Range  granite,  and  as  a  result  of  this 
intimate  intrusion  they  have  been  metamorphosed  to  their  present  condition 
of  mica-  and  hornblende-schists  and  gneisses. 

Relations  to  ilie  gahbro. — Within  the  limits  of  the  map  (PI.  II)  the 
Giants  Range  granite  and  the  great  Keweenawan  gabbro  are  in  proximity 
to  each  other  only  along  the  Kawishiwi  River,  from  sec.  34,  T.  63  N.,  R. 
10  W.,  to  sec.  19,  T.  63  N.,  R.  9  W.  Although  the  area  lying  within  the  map 
in  which  the  granite  and  gabbro  lie  close  together  is  small,  nevertheless  the 
relations  between  the  two  rocks  are  sufficiently  clear.  The  gabbro  lies 
obliquely  across  the  northeastern  continuation  of  the  Giants  Range  granite 
and  even  overlaps  the  Archean  greenstones  and  the  Lower  Hurouian 
sediments,  which  lie  north  of  the  granite.  The  way  in  which  these  rocks  are 
interrupted  in  their  eastern  continuation  indicates  an  eruptive  relationship 
of  the  gabbro  to  them.     Moreover  that  this  is  the  true  relationship  between 


"The  Mesabi  iron-bearing  district  of  Minnesota,  by  C.  K.  Leith:  Mon.  U.  S.  Geol.  Survey  Vol. 
XLIII,  1903,  p.  186.  , 


358  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  g-abbro  and  the  Giants  Range  granite  is  couchisively  shown  b}-  the 
jDi-esence  of  a  dike  of  gabbro  which  occurs  in  the  Giants  Range  granite  upon 
the  portage  at  the  falls  of  the  Kawishiwi,  in  sec.  19,  T.  63  N.,  R.  9  W.  This 
dike  was  not  directly  connected  with  the  gabbro,  but  is  macroscopically  the 
same  and  is  only  about  a  quarter  of  a  mile  away  from  the  contact  line 
between  the  gabbro  and  granite  massives.  Attention  may  be  called  to  one 
further  fact  indicative  of  the  intrusion  of  the  granite  by  the  gabbro,  and 
that  is  that  the  Giants  Range  granite  near  its  contact  with  the  Duluth 
gabbro  appears  a  little  more  basic  than  elsewhere,  and  approaches  the 
gabbro  somewhat  in  general  appearance.  Thus  the  granite  is  found  to 
contain  some  augite.  There  seems  in  many  places  to  be  a  transition 
between  these  two  rocks,  although  ordinarily  the  contact  is  sharp.  The 
transition  rock  is  in  a  few  places  broken  into  round  masses  and  these  are 
cemented  together  by  a  schistose  chloritic  material.  One  might  conceive 
of  such  a  transition  phase  being  equally  likely  to  result  from  the  intrusion 
of  the  gi'anite  into  the  gabbro.  However,  the  above-mentioned  dike  seems 
to  clinch  the  relationship  existing  between  these  two  rocks. 

AGE. 

From  the  preceding  statement  of  facts  concerning  the  relation  of 
the  Giants  Range  granite  to  the  adjacent  rocks,  we  are  enabled  to  draw 
the  conclusion  that  the  granite  was  intruded  after  the  Lower  Huronian 
sediments  were  deposited  and  before  the  intnision  of  the  Duluth  gabbro 
of  Keweenawan  age. 

FOLDING. 

As  has  been  already  intimated,  the  granite  shows  very  slight  effects  of 
crushing  or  of  the  action  of  mountain-building  forces,  but  that  it  has  been 
exposed  to  a  certain  amount  of  pressure  is  clearly  shown.  Small  granite 
dikes  cut  through  the  adjacent  greenstone  areas  and  lie  parallel  to  the 
schistosity  of  the  greenstones.  They  were  probably  intruded  subsequent 
to  the  production  of  this  schistosit}^,  and  hence  followed  along  the  planes  of 
easiest  parting,  which  were  the  planes  of  fissilitj^  in  the  rock.  Subsequent 
to  their  intrusion  these  granite  dikes  have,  however,  been  squeezed,  for  we 
find  that  occasionally  they  have  been  broken  into  pieces  and  these  broken 
portions  have  been  separated,  and,  in  fact,  in  some  cases  even  the  fragments 
of  the  dikes  have  been  rounded   until  the}'  have  acquired  a  more   or  less 


THE  LOWER  HURONIAN.  359 

lenticular  shape.  These  effects  are  probably  due  to  minor  folding,  which 
has  taken  place  subsequent  to  the  period  when  the  schistosity  was  produced 
and  the  granite  intruded,  this  second  period  of  folding  ha^dng,  indeed, 
emphasized  the  schistosity  due  to  the  first  period  of  folding  and  having 
slightly  affected  the  Giants  Range  granite. 

METAMORPHIC  ACTION   OF  THE   GIANTS   RANGE  GRANITE. 

The  metamorphic  effect  of  this  granite  on  the  greenstones  being  essen- 
tially similar  to  that  produced  by  the  intrusion  of  the  granite  of  Trout, 
Burntside,  and  Basswood  lakes  will  not  here  be  discussed,  the  reader  being 
referred  to  tlie  previous  discussion  for  the  details  and  results  of  the  process. 

As  a  result  of  the  intrusion  of  the  Giants  Range  granite,  the  Lower 
Huronian  sediments  have  also  been  extensively  metamorphosed.  A  dis- 
cussion of  this  metamorphism  may  be  found  under  the  discussion  of  these 
sediments  (p.  340). 

INTERESTING  LOCALITIES. 

Belations  to  the  Archean  Ely  greenstone. — The  relations  of  the  Giants 
Range  granite  to  the  Ely  greenstones  are  well  shown  in  the  SE.  \  of  sec. 
24,  T.  62  N.,  R.  13  W.,  and  in  the  NW.  \  of  sec.  19,  T.  62  N.,  R.  12  W. 

Relations  to  Soudan  formation. — Dikes  of  the  Giants  Range  granite 
have  been  observed  cutting  the  Soudan  formation  at  several  places;  for 
instance,  at  200  paces  west  of  the  southeast  corner  of  sec.  7,  T.  62  N.,  R. 
12  W.,  the  iron  formation  is  cut  by  injections  of  granite  which  run  parallel 
to  the  bands  of  the  iron  formation.  Dikes  of  the  same  granite  cut  the 
adjacent  Ely  greenstone  just  north  and  west  of  this  locality.  At  1,050 
paces  north,  550  prxes  west  of  the  southeast  corner  of  sec.  28,  T.  62  N., 
R.  13  W.,  the  iron  formation  is  likewise  cut  by  granite  dikes.  Similar 
dikes  cut  thts  iron  formation  in  sees.  3  and  4,  T.  62  N.,  R.  12  W.  These 
dikes  can  be  well  observed  at  the  places  where  they  occur  on  the  bare  hills 
south  of  Ely.  In  this  particular  instance  the  dikes  are  only  a  very  short 
distance  away  from  the  edge  of  the  main  mass  of  Giants  Range  granite, 
and  that  they  are  offshoots  from  it  can  hardly  be  doubted. 

Metamorphism  caused  hy  granite. — Reference  has  already  been  made  to 
the  fact  that  the  Giants  Range  granite  has  had  an  important  metamorphos- 
ing effect  on  the  rocks  which  it  has  intruded.  Its  effect  upon  the  Lower 
Huronian  slates  is  well  shown  in  the  area  south  of  the  Kawishiwi  River,  in 


360  THE  VERMILION  lEON-BEARING  DISTRICT. 

sec.  31,  T.  63  N.,  R.  10  W.,  and  in  sec.  36,  T.  63  N.,  R.  11  W.  In  fact,  at  a 
o-reat  number  of  places  in  this  area  a  traverse  almost  anywhere  will  show  it. 
The  granite  dikes  occasionally  occur  near  the  river,  but  become  more  and 
more  numerous  as  one  proceeds  southward  and  approaches  the  contact  of 
the  main  mass  of  granite.  For  the  most  part  the  sedimentar}-  character  of 
the  rocks  can  not  be  readily  recognized,  as  they  have  already  been 
metamorphosed  into  hornblende-  and  mica-schists.  At  one  place  on  the 
portage  leading  southeastward  to  Clearwater  Lake,  proof  of  the  sedi- 
mentary character  of  these  rocks  may  be  seen.  They  are  well-banded 
amphibole-  and  mica-schists,  but  a  few  bands  having  a  distinctly  conglom- 
eratic nature  were  observed,  although  even  these  fine-grained  conglomer- 
ates are  now  schistose  and  carry  a  good  deal  of  mica,  e^^dently  of 
secondary  origin.  Farther  south  the  granite  dikes  become  more  numerous 
and  the  metamorphism  more  extreme,  so  that  practically  the  banding  and 
the  connection  with  the  rocks  showing  ^^nquestioned  clastic  characters  are 
the  onlv  indications  of  the  sedimentary  nature  of  these  rocks. 

The  intrusive  relations  which  the  Giants  Range  granite  bears  to  the 
Ely  greenstone  are  shown  at  a  great  number  of  places  in  that  portion  of 
the  Vermilion  district  adjacent  to  the  area  underlain  by  the  Giants  Range 
granite.  It  is  hardly  necessary  to  enumerate  the  places  at  which  offshoots 
from  this  granite  penetrate  the  adjacent  greenstone,  as  they  may  be  found 
at  almost  any  place  in  the  portion  of  the  area  outlined  in  which  large 
exposures  exist.  Numerous  dikes  of  the  Giants  Range  granite  have  been 
found  in  nearly  every  place  where  its  contact  has  been  followed.  Manj'  of 
them  are  indicated  upon  the  accompanjdng  map  (PI.  II),  but  it  has  been 
found  impossible  to  show  all  of  those  which  have  been  found.  Numbers  of 
them  were  seen  in  sees.  24,  27,  and  28,  T.  62  N.,  R.  13  W.  Many  others 
occur  in  sees.  7,  8, 17,  18,  and  19,  T.  62  N.,  R.  12  W.,  and  they  are  especially 
easy  to  find  on  the  bare  hills  southeast  of  Ely,  in  sees.  1,2,  and  3,  T.  62 
N.,  R.  12  W.  A  number  of  such  dikes  cut  the  hills  south  of  Pickerel  Lake, 
along  the  line  between  sec.  2.5,  T.  63  N.,  R.  11  W.,  and  sec.  30,  T.  63  N., 
R.  10  W.  The  bold  hills  on  the  north  shore  of  the  Kawishiwi,  in  sees. 
20,  28,  and  29,  show  a  number  of  these  dikes  penetrating  the  greenstones. , 
Others,  in  considerable  number,  may  be  found  on  the  hills  north  of  Stone 
Lake,  in  sees.  16,  17,  20,  and  21,  T.  63  N.,  R.  10  W.  A  number  of  others 
likewise  occur  in  sees.  10,  14,  and  15,  T.  63  N.,  R.  10  W.,  and  at  a  number 
of  places  which  it  is  not  necessary  to  enumerate.     Throughout  this  part  of 


THE  LOWER  HURONIAN.  361 

the  district  the  relation  of  the  granite  to  the  greenstone,  which  is  shown  not 
only  by  the  presence  of  these  dikes  but  also  by  the  effect  of  the  granite  on  the 
greenstone,  may  be  seen,  if  strict  attention  is  paid  to  the  changes  which  take 
place,  as  the  granite  is  approached  from  the  north.  In  every  instance  where 
the  exposures  are  sufficiently  numerous,  as,  for  example,  in  the  vicinity  of 
Ely,  it  will  be  seen  that  the  greenstone  changes  its  character,  becoming  more 
and  more  schistose,  and  finally  passing  into  marked  amphibole-  and  mica- 
schists  in  close  proximity  to  the  granite. 

SKOWBASTK  GRAHSriTE. 

DISTRIBUTION,   EXPOSURES,  AND  TOPOGRAPHY. 

The  Snowbank  granite  has  received  its  name  from  the  fact  that  it  is 
typically  developed  around  Snowbank  Lake,  which  covers  several  sections 
and  parts  of  sections  of  T.  64  N.,  Rs.  8  and  9  W.,  and  T.  63  N.,  R.  9  W.  The 
granite  is  confined  exclusively  to  the  immediate  vicinity  of  the  lake,  being 
best  developed  on  the  southern  shores  of  the  lake  and  on  the  islands  in  that 
portion  of  the  lake. 

The  exposures  are  very  numerous  and  excellent.  Although  the  granite 
occupies  the  center  of  a  structural  anticline,  it  nevertheless  does  not  empha- 
size this  structure  by  forming  an  area  of  topographically  higher  ground 
than  that  of  the  surrounding  country.  The  topogi'aphy  is  not  of  marked 
character,  having  the  usual  rounded  gentle  contours  so  common  in  the 
glaciated  granite  areas  of  this  district. 

PETROGRAPHIC  CHARACTERS. 

The  granite  occm-s  in  good  exposures  in  the  vicinity  of  the  shores  of 
the  lake,  where  its  characters  may  be  easily  studied.  It  is  predominantly 
pink  to  red  in  color,  although  on  fresh  fracture  it  is  a  gray  or  flesh-colored 
rock.  Some  facies,  however,  are  much  darker  colored,  as  the  result  of  a 
higher  content  of  the  dark  minerals  than  is  contained  in  the  normal  granite. 
The  granite  varies  from  the  fine-grained  to  the  coarse-grained  form,  the 
medium-grained  facies  being  most  abundant.  Porjihyritic  facies  of  the 
granite  also  occur,  but  are  not  very  abundant.  The  porphyries  are  developed 
as  granite-porphyries  and  microgranite-porjihyries  with  feldspar,  augite,  and 
quartz  phenocrysts  in  a  fine-grained  groundmass.  These  porphyries,  as 
well  as  the  very  fine-grained  granites,  occur  chiefly  as  offshoots  from  the 


362  THE  VERMILION  IRON-BEARING  DISTRICT. 

main  medium-gi-ained  mass,  and  penetrate  the  sediments  wliicli  smTound 
the  granite  massive. 

Miueralog-ically  the  Snowbank  granite  varies  from  a  normal  mica-  and 
hornblende-granite  to  an  augite-granite,  and,  by  loss  of  quartz,  to  a  syenite. 
The  hornblende-granites  are  invariably  much  darker  than  the  mica-granites. 
These  last  tend  to  reddish  colors,  while  the  hornblende-granites  are  usually 
dark  gray  or  red  if  the  orthoclase  is  very  prominent  and  considerably 
weathered.  As  the  green  augite  takes  the  place  of  the  hornblende,  these 
hornblende-granites  pass  over  into  the  augite-granite.  The  augite-granite 
is  a  grayish,  flesh-colored  to  red  medium-grained  granite,  and  does  not 
differ  materially  from  the  normal  Snowbank  mica-  and  hornblende-granite  in 
macroscopic  appearance.  The  red  augite-granite  has  been  observed  to  cut 
the  normal  Snowbank  granite,  as  for  instance  on  the  point  projecting  north- 
eastward from  the  mainland  and  forming  the  NW.  ^  of  the  SW.  ^  of  sec.  36, 
T.  64  N.,  R.  9  W.  Here  the  medium-gi-ained  red  augite-granite  cuts  a 
hornblende-syenite  phase  of  the  Snowbank  granite,  and  is  in  its  turn  cut  by 
a  basalt  dike.  This  red  augite-granite  is  also  reported  to  be  cut  by  the 
hornblende-granite  "  Both  observations  are  correct,  the  ex^ilanation,  as  it 
appears  to  me,  being  that  they  are  of  essentially  the  same  age  and  are 
differentiation  products  of  the  same  magma.  For  this  reason  they  are 
included  here  together  as  constituting  the  Snowbank  granite  complex. 

No  attempt  has  been  made  to  discriminate  on  the  map  between  the 
normal  mica-granite,  the  hornblende-granite,  and  the  augite-granite.  They 
show  nothing  of  peculiar  interest  under  the  microscope.  The  normal 
combination  is  mica,  quartz,  and  feldspar,  both  orthoclase  and  plagioclase, 
with  some  iron  oxide  in  very  small  quantities.  Hornblende  is  commonly 
found  with  the  mica,  and  as  it  increases  in  quantity  the  rock  passes  to  a 
hornblende-granite.  Usually  a  great  deal  of  sphene  is  present  in  the 
hornblende- granite,  and  it  is  more  prominent  in  the  hornblende-syenites, 
which  are  connected  with  the  hornblende-granites  and  diifer  from  them 
only  in  containing  very  much  less  or  practically  no  free  quartz.  Augite 
accompanies  the  hornblende  in  some  of  the  hornblende-granites,  and  as  it 
increases  in  quantity  and  the  hornblende  diminishes  there  is  the  gxadation 
to  the  augite-granite. 

"Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV,  1899,  pp.  427-428. 


THE  LOWER  HURONIAN.  363 

RELATIONS  TO   ADJACENT  FORMATIONS. 

Relations  to  the  Loiver  Hiironian. — The  Snowbank  granite  is  found  in 
contact  with  the  adjacent  Lower  Huronian  sediments,  both  the  Ogishke  con- 
glomerates and  the  Knife  Lake  slates.  In  a  number  of  places  dikes,  offshoots 
from  this  mass,  occur  in  the  adjacent  sediments.  Moreover,  the  sediments 
near  the  granite  have  been  very  much  changed.  They  are  full  of  mica  in 
relatively  large  crystals,  and  in  general  the  rocks  have  been  recrystallized 
until  they  are  now  in  places  mica-schists.  The  crystalline  character  of 
these  rocks  is  most  noticeable  near  their  contact  with  the  main  granite  mass 
and  at  places  where  they  are  cut  through  by  numerous  dikes  of  the 
granite  and  where  the  fragments  of  the  sediments  are  inclosed  in  the  dike 
rocks.  The  farther  away  from  this  contact  we  go  the  less  numierous  the 
dikes  become  and  the  less  pronounced  are  the  indications  of  metamorphism 
until,  at  a  distance  varying  in  places  from  half  a  mile  to  a  mile,  the 
sediments  seem  to  show  their  normal  character.  The  presence  of  the  dikes 
in  the  sediments  and  the  contact  effect  of  the  granite  on  the  sediments 
clearly  show  the  intrusive  character  of  the  granite.  The  facts  referred  to 
briefly  above  were  observed  and  recorded  in  their  notebooks  by  the 
members  of  the  Minnesota  survey,  but  the  interpretation  g-iven  to  these 
facts  by  the  State  geologist"  is  very  different  from  that  given  above. 
According  to  him  the  Snowbank  granite  is  a  product  of  the  metamorjjhism 
of  acid  sediments,  graywackes,  and  conglomerates,  and  the  granite  and 
granite-porphyries  are  connecting  links  showing  transitions  to  the  sedi- 
ments. The  complete  fusion  of  these  graywackes  produced  the  granite. 
Incomplete  fusion  accounts  for  the  metamorphosed  sediments  surrounding 
the  granite  massive.  The  dikes  in  the  sediments  at  some  distance  from  the 
border  of  the  granite  are  portions  of  the  molten  sediments  which  penetrated 
the  unfused  ones. 

Relations  to  the  Keweenawan  gabhro. — The  granite  has  not  been  found 
in  actual  contact  with  the  large  mass  of  Duluth  gabbro  lying  south  of 
and  next  to  it.  Along  the  contact  there  is  a  slight  topographic  break, 
occupied  by  low  ground,  in  which  exposures  are  wanting.  The  granite 
is  of  Lower  Huronian  age,  and  there  can  be  no  doubt  that  the  Duluth 
gabbro    is    younger   than  it  is.     However,  if  the    gabbro   exercised  any 

"Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV,  1899,  pp.  287-294. 


364  THE  VERMILION  IRON-KEARING  DISTRICT. 

metamorphic  effects  upon  the  granite,  they  have  not  been  observed,  nor 
have  any  dikes  that  could  be  traced  to  the  gabbro  been  found  cutting 
through  it. 

INTERESTING  LOCALITIES. 

The  best  portion  of  the  area  in  the  vicinity  of  Snowbank  Lake  in  which 
to  study  the  relations  of  this  granite  to  the  adjacent  sediments  is  east  of  the 
lake,  in  general  between  its  outlet  and  Disappointment  Lake.  In  this  area 
the  hills  are  bare,  and  good  exposures  are  numerous.  A  number  of  good 
exposures  can  be  found  on  the  shores  of  Snowbank  Lake  north  and  south 
of  its  outlet,  and  here  the  Snowbank  granite  is  found  cutting  the  adjacent 
conglomerates  in  numerous  dikes.  These  same  relationships  ma}"  be  found 
still  farther  inland,  on  the  hills  in  the  area  to  which  reference  has  already 
been  made.  Other  exposures  showing  the  same  relations  may  be  found  at 
almost  any  place  along  the  north  and  northwest  shores  of  the  lakes,  and 
a  traverse  inland  from  these  shores  will  almost  invariably  result  in  the 
finding  of  dikes  cutting  the  adjacent  schist.  There  is  nothing  especially 
peculiar  in  the  relations  of  these  dikes  to  the  rocks  which  they  cut  or  in 
the  occurrence  and  appearance  of  these  dikes,  consequently  no  detailed 
enumeration  of  them  will  be  given. 

CACAQtTABIC  GRAKITE. 

This  granite  occurs  typically  on  the  islands  in  and  the  shores  of 
Cacaquabic  Lake,  from  which  it  was  named  by  the  Minnesota  survey.  It 
has  been  more  or  less  extensively  studied  by  the  Minnesota  State  geologist 
and  his  assistants,  and  mention  and  description  of  it  occur  in  a  number  of 
the  State  reports." 

U.  S.  Grant  has  made  an  especially  detailed  study  of  this  granite,''  and 
as  a  result  of  his  careful  mineralogic  and  chemical  analyses  has  deterinined 
it  as  one  of  the  comparatively  rare  augite-soda  granites.  Studies  of  the 
Cacaquabic  granite  corroborate  in  the  main  the  statements  of  Grant,  l)ut 
there  has  been  no  opportunity  to  make  detailed  mineralogic  studies  or 
chemical  analyses  of  the  rock,  and  data  resulting  from  these  have  been 
obtained  from  Grant's  articles,  to  which  reference  has  been  made. 

«Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Fifteenth  Ann.  Kept.,  1886,  pp.  361-369;  Sixteenth 
Ann.  Kept.,  1887,  pp.  149-1.56;  Twentieth  Ann.  Kept.,  1891,  pp.  70-79;  Twenty-first  Ann.  Kept.,  189L', 
pp.  5-59,  2  plates.  Grant,  U.  S.,  Am.  Geologist,  Vol.  XI,  1893,  i)p.  383-388;  Geol.  and  Nat.  Hist. 
Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  pp.  294,  442,  and  450;  Final  Kept.,  Vol.  V,  1900, 
descriptions  of  sections  in  various  places. 

''Loc.  cit. 


THE  LOWER  HURONIAN.  365 

DISTRIBUTION,  EXPOSURES,  AND   TOPOGRAPHY. 

The  granite  is  confined  in  its  distribiition  to  the  immediate  vicinity  of 
Cacaqiiabic  Lake,  and  is  especially  well  developed  on  the  southern  side. 
It  extends  back  from  the  lake  for  a  maximum  distance  of  aboxit  a  mile, 
reaching  down  into  the  SE.  ^  of  sees.  1  and  2,  T.  64  N.,  R.  7  W.  The 
granite  occupies  on  the  whole  higher  ground  than  is  occupied  by  the 
adjacent  rocks,  forming,  especially  in  sees.  1  and  2,  fairlj-  high  hills.  There 
are  excellent  exposures  on  a  number  of  islands  in  the  lake  and  also  on  the 
mainland  near  the  shore. 

PETROGRAPHIC  CHARACTERS. 

The  Cacaquabic  granite,  like  the  other  granite  complexes  of  the  Ver- 
milion district,  shows  considerable  variation  both  in  texture  and  mineralogic 
composition.  The  main  mass  of  the  granite  is  a  medium-grained  gray  or 
pink  to  red  rock,  whereas  on  the  periphery  of  the  gi'anite  area  the  finer- 
gi'ained  and  granite-porphyry  facies  are  developed. 

The  granite-porphyry  facies  contains  large  phenocrysts  of  plagioclase 
feldspar  lying-  in  a  fine-grained  gray  to  red  groundmass.  The  phenocrj'sts 
themselves  range  from  gray  to  red  in  color,  depending  on  the  degree  and 
the  character  of  the  alteration.  When  red  in  part  or  as  a  whole,  the 
phenocrysts  give  the  gTanite-porphyry  a  very  striking  appearance.  They 
then  stand  out  prominently  from  the  lig'hter-colored  groundmass.  In  general 
there  seemed  to  be  no  an-angement  of  the  phenocrysts,  but  in  one  case  the 
phenocrysts  of  the  granite-porphyry  did  show  a  distinct  parallelism  of  their 
major  dimensions.  There  was  thus  produced  a  more  or  less  perfect 
macroscopic  flowage  structure. 

The  granite  and  granite-porphyry  are  massive,  although  in  places 
much  jointed  and  separated  into  small  blocks  by  the  joint  planes.  In 
places"  the  fractures  in  the  granite  are  filled  by  veins  of  infiltrated  quartz. 

Mineralogically  the  granite  varies  considerably.  While  the  main  mass 
is  an  augite-soda  granite,''  an  examination  of  the  specimens  collected  shows 
variations  to  a  hornblende-gTanite  and  hornblende-mica-gTanite.  Grant 
reports  an  aug-ite-biotite-syenite  facies."      The    minerals  constituting   the 


«The  Geology  of  Kekekabic  Lake,  by  U.  S.  Grant:  Geol.  and  Ivat.  Hist.  Survey  of  Minnesota, 
Twenty-first  Ann.  Eept.,  1892,  pp.  34-36. 
''Loc.  cit.,  p.  33. 
<^Loc.  cit.,  p.  50. 


3(36  THE  VERxMlLlON  IRON-JJEARING  DISTRICT. 

granite  are  quartz,  feldspar  (both  orthoclase  and  plagioclase),  augite,  horn- 
blende, mica,  sphene,  apatite,  magnetite,  and  hematite.  The  phenocrysts  are 
of  plagioclase.  Most  of  the  minerals  have  their  normal  characters  and  will 
not  be  described.  The  feldspar  and  angite  are  somewhat  exceptional,  and 
have  been  described  in  detail  by  Mr.  Grant,"  from  whose  description  the 
following  details  concerning  these  are  taken. 

Gi-ant's  study  of  the  feldspar  shows  that  the  plagioclase  has  a  specific 
gravit}^   ranging  from  2.58  to  2.62  and  the  chemical  composition  shown 

below.'' 

Analysis  of  feldspar  of  Cacaquahic  granite. 

Per  cent. 

SiOj 67.99 

AlA  19.27 

FeA 82 

CaO  75 

MgO 02 

K2O : 3.05 

Na^O 6.23 

H,0 90 

Total 99.03 

From  this  he  concludes  that  the  feldspar  is  an  anorthoclase  composed 
of  orthoclase,  albite,  and  anorthosite  molecules  in  the  following  propor- 
tions: Org,  Abi4,  Aui. 

The  augite  varies  from  a  colorless  one  to  a  variety  having  a  bottle- 
green  color.  The  colorless  kind  is  not  uncommonly  intergrown  with  the 
green  variety.  Sometimes  this  is  in  zonal  intergrowth.  The  individuals 
possess  a  very  good  crystallographic  development.  The  specific  gravity  of 
the  augite  as  a  whole  is  somewhat  higher  than  3.  An  analysis  of  the 
augite  given  by  Grant  is  here  quoted: 

Analysis  of  augite  of  Cacaqimbic  granite. 

Per  cent. 

SiO, 53.19 

Al^O, 2.  38 

FeA 9-5 

FeO 315 

CaO : 17.81 

MgO 9- -13 

K,0 38 

Na,0 2.(« 

H,0 01 

Total 100.23 

"Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Twenty-tirst  Ann.  Kept.,  1892,  pp.  47-48.     i-Ibid.,  p.  44. 


THE  LOWER  HURONIAN. 


367 


Assuming-  that  this  represents  an  isomorphous  mixture  of  the  diopside,  hedden- 
bergite,  acmite,  and  fassaite  molecules,  and  calculating  their  relative  proportions, 
we  get  approximateh^  the  following  result : 

Per  cent. 

Diopside,  Mg  Ca  SijOj 47 

Heddenbergite,  Ca  Fe  SijO^ 27 

Acmite,  Jsa  Fe  SijOg 21 

Fassaite,  Mg  AUSiOs 5 

In  the  considerable  percentage  of  the  acmite  molecule  this  augite  approaches  in 
composition  the  pyroxene  of  the  more  alkaline  rocks,  the  eleolite  syenites.  This 
analysis  very  probabh^  represents  quite  well  the  usual  composition  of  the  green 
augite,  as  the  proportion  of  zonal  crystals,  with  colorless  centers,  and  entire  colorless 
crystals,  is  small.  The  colorless  augite  is  very  similar  to  that  of  the  well-known 
augite  granite  from  Laveline  in  the  Vosges. 

The  analyses  of  the  auorthoclase  feldspar  and  of  the  augite  which  are 
quoted  above  show  such  a  proportion  of  sodium  oxide  that  one  would 
expect  the  granite  itself  to  exhibit  a  very  high  ratio  of  that  substance.  This 
proportion  is  shown  in  the  following  analyses,"  I  being  that  of  the  normal 
Cacaquabic  granite  facies  as  given  by  Grant,  and  II  being  that  of  the 
granite-porphyry  facies. 

Analyses  of  Cacaquabic  granite  and  granite-porphyry. 


SiO,.. 
TiO^  . 
P2O5.. 
Al.Oa- 
Fe^Os. 
FeO.. 
MnO. 
CaO.. 
MgO. 

k;o.. 

Na^O. 
H,0.. 


Per  cenl. 
66.84 


II. 


Per  cent. 
67.42 


Tr. 

18.22 
2.27 
0.20 


3.31 
0.81 
2.80 
5.14 
0.46 


Total. 


100. 05 


0.07 
15.88 
1.37 
1.14 
Tr. 
3.49 
1.43 
2.65 
6.42 
0.05 


99.92 


«Loc  cit.,  p.  41. 


368  THE  VEKMILION  IRON-BEARING  DISTRICT. 

Secondary  minerals,  such  as  sericite,  chloiite,  and  hornblende  occui- 
abundantly  in  some  of  the  rocks.  The  rocks  have  locally  been  crushed, 
but  the  crushing-  has  in  no  case  reached  such  a  degree  as  to  produce  schis- 
tose rocks,  although  the  microscope  shows  crushing  effects  reaching  even  to 
the  granulation  of  the  quartz.  The  microscopic  fractures  are  healed  by 
quartz  and  feldspar  and  also  by  hornblende  when  the  fractures  cross  a  horn- 
blende individual.  In  such  a  case  one  can  readily  distinguish  the  second- 
ary needles  of  nearly  colorless  hornblende  which  traverse  the  fractures  and 
unite  the  pieces  of  the  old  massive  green  hornblende  individual. 

RELATIONS   TO   ADJACENT  FORMATIONS. 

Relations  to  the  Lower  Huronian. — The  Cacaquabic  gi-auite  lies  adjacent 
to  the  Lower  Huronian  sediments,  which  very  nearly  surround  it.  In  the 
area  in  which  the  sediments  are  wanting  the  gabbro  Hes  next  to  the  granite. 
The  relations  of  the  gi-anite  to  the  adjacent  formations  are  not  nearly  so 
clear  here  as  were  the  relations  described  for  the  Snowbank  granite.  The 
granite  contains  dark-colored  chloritic  basic  inclusions  which  may  have 
been  brought  up  from  the  Archean  greenstone  that  underlies  the  sediments, 
an  anticline  of  it  lying  a  short  distance  southeast  of  the  granite,  and  another 
north  of  it.  The  Cacaquabic  granite  grows  finer  grained  as  the  mantle  of 
sediments  is  approached,  and  it  has  been  found,  moreover,  cutting  the  sedi- 
ments in  dikes  which  show  clearly  that  it  is  intrusive  in  them  and  of 
younger  age.  If  it  has  metamorphosed  them,  as  is  probably  the  case,  its 
metamorphism  is  concealed  by  the  later  metamorphism  caused  by  the  gabbro. 

Belations  to  the  gabbro. — The  statement  that  the  gabbro  is  later  than  the 
gi-anite  is  not  warranted  by  any  actual  contacts  that  have  been  found 
between  them,  or  by  the  occurrence  of  any  inclusions  of  the  granite  in  the 
gabbro,  but  is  based  chiefly  on  their  field  relations,  shown  on  the  nccom- 
panying  map,  and  on  the  comparatively  youthful  age  of  the  gabbro.  Thus 
it  will  be  seen  that  the  granite  everywhere,  except  on  the  southeastern 
edge,  is  suiTOunded  by  the  sediments.  Presumably  it  was  originally 
completely  surrounded  by  them:  but  for  this  narrow  area,  however,  the 
gabbro  has  cut  out  the  sediments  and  overlaps  on  tlie  area  underlain  by 
granite. 


THE  LOWER  HURONIAN.  369 

AGE. 

It  will  thus  be  seen  that  the  Cacaquabic  granite  massive  is  an  intrusive 
of  youup'er  ao-e  than  the  Knife  Lake  slates,  but  older  than  the  Keweenawan 
gabbro. 

INTERESTING  LOCALITIES. 

At  only  two  places  has  the  granite  been  found  showing  its  relations  to 
the  adjacent  sediments.  One  of  these  places  is  700  paces  north,  650  paces 
west  of  the  southeast  corner  of  sec.  1,  T.  64  N.,  R.  7  W.,  south  of  Caca- 
quabic Lake,  where  the  sediments  are  cut  by  dikes  of  the  granite.  ■  The 
other  point  is  just  back  of  the  southwest  shore  of  Cacaquabic  Lake  south 
of  a  large  granite  island.  Here  the  porph3'ritic  form  of  the  granite  cuts 
the  adjacent  conglomeratic  sediments. 

Still  other  dikes  occur  in  the  southeast  corner  of  sec.  1,  T.  64  K., 
R.  7  W.,  on  the  hills  north  of  the  small  lake  in  which  the  four  sections 
which  meet  at  this  place  have  their  corner. 

VARIOUS  ACID  DIKES. 

Certain  acid  dikes,  to  which  reference  has  already  been  made,  are 
found  cutting  through  the  various  formations  of  the  district,  but  can  not 
be  directly  connected  with  any  of  the  large  eruptive  masses.  They  are 
presumed  to  be  of  Lower  Huronian  age,  and  the  evidence  for  this  will 
be  given  on  another  page,  yet  it  is  possible  that  some  of  them  may 
be  of  Keweenawan  age,  although  if  there  are  any  Keweenawan  dikes 
included  among  them,  they  can  not  be  recognized  as  such  on  account  of 
lack  of  evidence.  They  are  not  of  sufficient  interest  or  importance,  how- 
ever, to  warrant  a  description  of  the  individual  dikes;  consequently  an 
attempt  will  be  made  to  give  merely  a  general  idea  of  their  characters. 

DISTRIBUTION. 

The  distribution  of  these  dikes  will  be  seen  on  the  maps  in  the  accom- 
panying atlas,  on  which  they  are  marked  with  the  same  symbol  and  the 
same  color  as  that  used  for  the  Griants  Range,  Snowbank,  and  Cacaquabic 
granites.  In  most  cases  the  exposures  of  these  dikes  are  small,  so  that  in 
many  instances  it  is  practically  impossible  to  state  absolutely  that  they  are 
younger  than  the  rocks  near  them.  This  presumption,  however,  is 
MON  XLV — 03 34 


370  THE  VERMILION  IRON-BEARING  DISTRICT. 

AYarranted  by  tlie  occuiTence  of  tiiese  other  rocks  which  suiTound  them  in 
numerous  exjDOSures.  They  are  of  such  small  area!  distribution  that  they 
do  not  materially  influence  the  topography  in  general,  although,  since  they 
are  generally  harder  than  the  rocks  through  which  they  cut,  they  do 
exert  a  local  influence  and  are  found  forming  knolls  or  ridges. 

PETROGRAPHIC  CHARACTERS 

Macroscopic. — These  dikes  vary  in  grain  from  very  fine-gi'ained,  almost 
cryptocrystalline  rocks,  to  those  which  may  be  classed  as  coarse-grained 
ones.  Moreover,  a  porphyritic  structure  is  of  very  common  occuiTence,  the 
quartz  being  sometimes  the  sole  phenocryst  though  at  other  times  both 
quartz  and  feldspar  occur  as  phenocrysts.  In  color  there  is  likewise  con- 
siderable variation.  For  the  most  part  the  rocks  are  gray  to  pink  on  fresh 
fracture.  Some  of  the  finer-grained  ones,  however,  are  dark  purplish.  The 
weathered  surfaces  are  nearly  always  pink  to  reddish.  The  dikes  vary 
greatly  in  width.  They  are  usually  narrow,  but  some  dikes  as  much  as  10 
feet  wide  have  been  observed,  and  the  grain  usually  varies  with  the  width, 
the  finer-grained  rocks  occurring  in  the  narrower  dikes  and  as  the  selvage 
of  the  wider  ones. 

3Iicroscopic. — Under  the  microscope  one  can  recognize  phenocrysts  of 
green  hornblende  in  association  with  phenocrysts  of  feldspar  and  quartz- 
The  quartz  is  relatively  scarce.  These  phenocr3^sts  lie  in  an  exceedingly 
fine-grained  groundmass  with,  in  some  cases,  grains  too  small  to  permit 
their  characters  to  be  recognized.  Generally  it  can  be  seen  that  the  ground- 
mass  is  made  up  of  quartz  and  feldspar,  flakes  of  chlorite  and  sericite,  and 
some  iron  ore.  Occasionally  the  grains  and  flakes  are  arranged  in  parallel 
lines  which  so  bend  around  the  phenocrysts  as  to  bring  out  a  very  well- 
marked  flowage  texture.  All  of  the  minerals  have  their  usual  characters, 
and  hence  no  description  will  be  given  of  them. 

RELATIONS  TO  ADJACENT  FORMATIONS. 

It  is  believed  that  these  acid  dikes  are  offshoots  from  tlie  various  granites 
described  in  this  chapter.  They  show  a  general  petrographic  similarity  to 
them.  Still  they  are  not  so  much  like  them  as  to  warrant  one  in  making  a 
positive  statement  that  they  are  the  same.  Moreover,  they  have  not  been 
connected  in  the  field,  nor  have  any  chemical  analvses  been  made  which 


THE  LOWER  HURONIAN.  371 

would  enable  one  to  determine  their  similarity  in  chemical  composition  to 
the  adjacent  granites.  The  differences  between  them,  which  are  textural, 
can  be  explained  by  the  fact  that  these  offshoots  occur  in  so  much  smaller 
quantity  that  naturally  they  would  not  acquire  the  same  textures  as  the 
coarser-grained  granites  and  granite-^oorphyries  of  the  large  massives. 

It  has  already  been  stated  that  these  acid  rocks  occur  as  dikes  in  the 
adjacent  formations.  In  some  instances  their  dike  character  is  not  clearly 
shown,  actual  contacts  between  them  and  the  rocks  occurring  nearest  to 
them,  and  tlu'ough  which  they  cut,  being  wanting.  From  the  fact  that 
they  are  igneous  rocks  and  more  or  less  completely  surrounded  by  other 
eruptives  or  by  sedimentaries,  they  are  supposed  to  be  intrusive  in  these. 
They  have  been  found  cutting  all  of  the  rocks  thus  far  described  from  the 
Vermilion  district,  those  of  eruptive  as  well  as  those  of  sedimentary  orig'in, 
with  the  exception  of  the  various  Lower  Huronian  granites.  They  are  con- 
sequently known  to  be  younger  than  all  of  the  rocks  which  they  cut.  Their 
relations  to  the  Upper  Huronian  sediments  and  to  the  great  Duluth  gabbro 
of  Keweenawan  age  have  not  been  determined  conclusively.  However, 
as  the  result  of  a  reconnaissance  in  the  Keweenawan  of  the  Lake  Superior 
region,  it  has  been  found  that  the  Keweenawan  is  cut  by  acid  rocks. 
While  these  have  not  been  connected  petrographically  with  the  dikes  in  the 
Vermilion  district,  nevertheless  it  may  be  well  to  suggest  the  possibility 
that  at  least  some  of  the  acid  dikes  in  the  Vermilion  correspond  to  those 
which  cut  through  the  Keweenawan  rocks. 

BASIC  AND  IlSrTEEMEDIATE  IjVTRUSIVJES  OF  LOWER   HURONIAN  AGE. 

At  various  places  in  the  Vermilion  district  basic  and  intermediate 
dikes  have  been  observed  cutting  the  country  rock.  These  can  be  easily 
divided  into  dikes  of  apparently  different  age  by  using  as  a  criterion  for 
this  the  difference  in  alteration.  This  macroscopically  determined  difference 
is  substantiated  by  the  condition  of  the  rocks  as  shown  by  microscopic 
examination.  On  the  one  hand  there  are  certain  dikes  which  are  composed 
of  very  fresh  dolerite  and  basalt  and  which  show  distinct  selvages.  These 
cut  through  all  the  other  rocks  of  the  Vermilion  district,  including  the 
gabbro.  Just  outside  of  the  district  are  dikes  identical  in  character  with 
these  and  cutting  even  the  acid  red  rock  of  the  Keweenawan,  which  itself  is 
known  to  cut  the  gabbro.     These  fresh  dikes  are  clearly  of  Keweenawan 


372  THE  VERMILION  IRON-BEARING  DISTRICT. 

or  post-Keweenawan  age,  and  will  be  described  under  the  beading 
"Keweenawan"  in  Chapter  VI.  On  the  other  hand  there  are  dike  rocks 
which  cut  all  the  rocks  of  Archean  and  Lower  Hurouian  age,  but  no 
definite  proof  has  been  obtained  that  they  i7itrude  any  of  the  rocks  younger 
than  these.  The  much  greater  age  of  these  dikes  is  shown  in  their  more 
extensive  alteration,  indicated  macroscopically  by  their  green  color,  and  by 
the  occasional  presence  of  an  imperfectly  developed  schistosity.  The  orig- 
inal characters  of  these  dike  rocks  are  rarely  well  enough  preserved  to  enable 
one  to  determine  positively  just  the  kind  of  rocks  they  are.  It  can  be  said 
in  general  that  they  are  of  basic  and  intermediate  character.  Some  have 
unquestionably  been  derived  from  dolerites  and  basalts.  Others,  it  is  clear, 
belong  to  the  lamprophyric  rocks. 

The  dolerites  and  basalts  are  invariably  very  much  altered.  Occasion- 
ally a  fairly  well-preserved  ophitic  texture  may  be  observed.  Usually, 
however,  all  textures  have  been  destroyed  as  the  result  of  orogenic 
movements,  and  the  rocks  have  become  fairly  schistose.  They  were 
evidently  intruded,  however,  in  the  Archean  rocks  after  the  latter  had  been 
subjected  to  pressure,  as  they  are  found  in  some  cases  to  have  been  injected 
parallel  to  the  schistosity  of  these  rocks.  The  usual  constituents  are  such 
as  are  commonly  found  in  these  altered  basalts:  actinolite,  chlorite,  apatite, 
calcite,  muscovite,  feldspar,  a  little  quartz,  sphene,  and  iron  oxides. 

The  lamprophyric  dikes  above  referred  to  occur  usually  in  very  narrow 
dikes  and  while  ordinarily  extremely  altered,  nevertheless  are  generally  not 
so  much  altered  as  are  the  dolerites.  These  rocks  consist  of  various 
combinations  of  plagioclase  and  orthoclase  feldspar,  with  biotite,  hornblende, 
augite,  and  iron  oxide.  The  hornblende  and  augite  are  the  predominant 
dark  silicates.  A  few  serpentinous  areas  indicate  the  former  presence  of 
olivine.  The  minerals  are  so  much  altered  that  a  trustworthy  separation 
of  these  rocks  into  the  various  divisions  of  the  lamprophyres  to  which  they 
belong  could  not  be  made.  There  seem  to  be  represented  among  them, 
however,  chiefly  biotite-kersantites,  augite-kersantites,  and  the  hornblende- 
and  augite- vogesites.     With  these  there  also  seem  to  be  some  camptonites. 

Certain  other  dikes  in  the  district  which  were  observed  were  so 
extremely  altered  that  one  could  only  see  that  the  original  rock  carried 
hornblende,  but  the  petrographic  position  of  these  dikes  could  not  be 
determined. 


THE  LOWER  HURONIAN.  373 

INTERESTING    LOCALITIES. 

No  attempt  will  be  made  here  to  enumerate  all  the  places  where  these 
dikes  occur.  A  number  of  them  have  been  shown  in  exaggerated  size  on 
the  accompanying  maps.  On  the  west  end  of  Stuntz  Island  there  is  a 
plexus  of  these  basic  dikes  cutting  the  Ogishke  conglomerate.  These  dikes 
run  parallel  to  one  another,  diverge  from  one  another  in  places,  and  at  other 
points  cut  one  another.  At  one  place  on  the  top  of  the  conglomerate  ridge 
just  south  of  the  small  northwestward-pointing  bay,  nine  dikes  essentially 
parallel  and  varying  in  width  from  IJ  inches  ujj  to  6  feet  were  counted 
within  a  distance  of  60  feet.  These  dikes  cut  right  across  the  fragments  in 
the  conglomerate,  giving  sharp  contacts.  The  jasper  fragments  in  the  con- 
glomerate which  have  been  cut  by  the  dikes  seem  to  have  been  apparently 
unaltered  by  them.  The  dikes  themselves  are  moderately  schistose,  espe- 
cially on  the  edges.  The  rock  of  these  dikes  is  extremely  altered  and 
appears  to  have  been  a  dolerite. 

Similar  dikes  can  be  seen  cutting  across  the  granite  island  just  east  of 
Stuntz  Island,  and  still  others  were  seen  on  the  high  hill  along  the  shore 
near  the  northeast  corner  of  sec.  22,  T.  62  N.,  R.  15  W. 

A  number  of  lamprophyric  rocks  were  observed  cutting  the  Ely  green- 
stone, the  Ogishke  conglomerate,  and  Knife  Lake  slates  near  the  center  of 
sec.  17,  T.  63  N.,  R.  9  W.  Similar  dikes  occur  on  the  hill  on  which  Ely 
is  built,  northeast  of  the  Methodist  church,  and  also  on  tlie  greenstone 
hills  on  the  north  side  of  Long  Lake. 


CHAPTER    V. 

THE  UPPER  HURONIAK  (ANIMIKIE). 

in  the  eastern  portion  of  the  Vermilion  district  there  has  lieen  fonnd 
overlying  the  Knife  Lake  slates  and  the  Ogishke  conglomerate  where  these 
are  present  and,  where  they  are  wanting,  lying  immediately  upon  the  Ely 
greenstone  or  the  granite  of  Saganaga  Lake,  a  series  of  sedimeutarj-  rocks, 
of  which  a  considerable  thickness  is  exposed.  These  rocks  belong  to  the 
great  sedimentary  series  which  is  best  developed  in  a  very  wide  area  lying 
along  the  northwest  and  north  shores  of  Lake  Superior  and  extending  well 
up  into  Canada,  but  which  is  also  well  developed  in  the  Mesabi  district  on 
the  southern  slope  of  the  Giants  range  in  .Minnesota.  To  this  series  the 
name  Auimikie,  the  Ojibway  word  for  thunder,  has  been  given  by  Hunt" 
from  the  fact  that  these  rocks  are  typically  developed  in  the  vicinity  of  the 
two  well-known  topographic  features  of  the  north  shore  of  Lake  Superior, 
namely  Thunder  Bay  and  Thunder  Cape. 

The  Upper  Huronian  series  of  the  Vermilion  district  may  be  readily 
divided  into  two  facies  of  rocks  which  are  quite  different  petrographically. 
At  the  bottom  of  the  series  occurs  an  iron-bearing  formation,  known 
as  the  Gruuflint  formation,  consisting  of  bands  of  ferruginous  carbonates, 
quartz,  magnetitic  quartz,  magnetitic  ore,  and  augite,  hypersthene,  horn- 
blende, olivine,  griinerite,  and  magnetite  rocks.  All  of  these  apparently 
represent  altered  forms  of  some  original  ferruginous  rocks.  Above  these 
iron-bearing  rocks  there  occurs  a  great  slate-graywacke  formation,  known 
as  the  Rove  slate. 

SECTION  I.— GtTNFLiINT  F0R»IAT10N. 

The  rocks  of  this  formation  are  well  developed  on  the  north  shore  of 
Gunflint  Lake,  from  which  their  name  has  been  derived.  They  extend  in 
a  belt,  shown  on  the  maps  in  the  accompanying  atlas,  for  a  number  of 


"The  geognostical  history  of  the  metals,  by  T.  Sterry  Hunt:  Trans.  Am.  Inst.  Min.  Eng.,  Vol.  I, 
pp.  3.31-395;  Vol.  II,  pp.  58-59. 

374 


UPPER  HURONIAN.  375 

miles  to  the  west  of  the  lake,  which  is  a  well-known  feature  of  the  inter- 
national boundary  route.  Rocks  with  which  these  are  correlated  occur  in 
the  Mesabi  district,  and  are  there  known  as  the  Biwabik  formation. 

DISTRIBUTION,   EXPOSURES,  AND   TOPOGRAPHY. 

Distribution. — The  iron  formation  has  a  wide  distribution  in  the  Mesabi 
district,  extending  through  it  from  end  to  end.  The  Gunflint  formation 
can  be  looked  upon  as  the  eastern  continuation  of  the  iron-bearing  Biwabik 
formation  of  the  Mesabi  district;  In  the  Vermilion  district  this  iron  forma- 
tion has  a  restricted  distribution.  The  area  in  wliicli  the  rocks  occur  is  in 
places  not  more  than  about  200  paces  wide,  from  this  spreading  out  to  a 
width  of  nearly  half  a  mile.  Northeast  of  Paulson's  mine,  in  sees.  21  and  22, 
T.  66  N.,  R.  4  W.,  there  is  an  east-west  tongue  of  the  Gunflint  rocks  project- 
ing westward  into  Ely  greenstone.  About  three-foui-ths  of  a  mile  east  of 
Paulson's  mine,  in  sec.  27,  T.  65  N.,  R.  4  W.,  where  a  great  north-south  valley 
cuts  directly  across  the  Gunflint  formation,  the  narrow  belt  of  iron  formation 
joins  a  wider  area  of  the  same  rocks,  which  extends  over  the  greater  portion 
of  sees.  23  and  26,  T.  65  N.,  R.  4  W.  The  Gunflint  formation  is  widest  in 
these  sections,  its  great  width  being  due  chiefly  to  the  fact  that  a  fairly  wide 
synclinal  fold  has  here  been  stripped,  leaving  exposed  an  unusuall}-  large 
area  of  the  iron  formation.  East  of  these  sections,  toward  the  international 
boundary,  the  formation  thins  down  rapidly.  The  northern  boundary  of 
the  Gunflint  formation  in  this  area  is  marked  by  the  Knife  Lake  slates  and 
Ogishke  conglomerate  of  the  Lower  Huronian  series,  and  the  Ely  green- 
stone and  granite  of  Saganaga  Lake  of  the  Archean.  This  is  the  order  in 
which  the  Gunflint  formation  comes  in  contact  with  these  rocks  from  west 
to  east.  This  is  also  the  stratigraphic  order,  passing  from  the  youngest  on 
the  west  to  the  oldest  on  the  east,  with  the  single  exception  of  the  granite 
of  Saganaga  Lake,  which  intrudes  the  greenstone. 

The  southern  boundary  of  the  iron-bearing  formation  over  the  greater 
portion  of  the  area  in  which  it  occurs  is  the  northern  edge  of  the  Duluth 
gabbro.  From  the  SW.  J  of  sec.  26,  T.  65  N.,  R.  4  W.,  however,  the  direc- 
tion of  the  southern  boundary  changes.  From  here  it  swings  northeastward 
and  the  Rove  slates,  which  overlie  it,  begin  to  appear  with  a  narrow  edge 
of  a  wedge  widening  to  the  east  and  separating  the  Gunflint  formation  from 
the  gabbro. 


376  THE  VERMILION  IRON-BEARING  DISTRICT. 

Exposures. — The  exposures  in  the  areas  outlined  are  very  good.  In 
all  instances  they  are  sufficient  to  enable  one  to  study  the  rocks  in  consid- 
erable detail  and  ti-ace  out  their  continuation  without  greater  difficulty 
than  is  offered  by  the  very  rough  and  thickly  timbered  character  of  the 
country. 

Topography. — Where  the  Gunflint  formation  occurs  in  sufficient  quantity 
to  aifect  the  topography  to  a  noticeable  extent,  the  forms  produced  in  it  are 
fairly  characteristic.  As  the  result  of  the  monoclinal  southward  di])  and 
the  differential  erosion  of  the  harder  and  softer  layers,  a  series  of  ridges  are 
produced  which  trend  about  east-west  and  have  very  steep  northward-facing 
escarpments  with  gentle  southerly  slopes  corresponding  approximately  to 
the  dip  of  the  beds.  It  may  be  well  to  mention  here  that  sills  of  dolerite 
lying  approximately  parallel  to  the  bedding  frequently  form  the  top  of  the 
larger  ridges.  This  same  kind  of  topography,  but  in  a  more  marked  form, 
is  also  developed  in  the  area  underlain  by  the  Rove  slates,  which  is  adjacent 
to  that  in  which  the  Gunflint  formation  occurs,  and  will  be  found  described 
in  greater  detail  elsewhere  (pp.  391-392). 

About  a  mile  east  of  Paulson's  mine  there  is  one  very  noticeable  topo- 
graphic feature  which  is  not  in  agreement  with  the  general  topography — 
a  large  cross  valley,  running  about  north  and  south,  which  appears  to  rep- 
resent an  old  pre-Glacial  valley  formerly  occupied,  perhaps,  by  a  fore- 
ranner  of  the  present  Cross  River,  which  flowed  through  it  on  its  way  north 
into  Boundary  River  and  Saganaga  Lake. 

STRUCTURE. 

The  structure  of  the  Gunflint  formation  in  that  ^^ortion  which  is 
exposed  in  the  Vermilion  district  is  not  very  complicated.  There  is  a 
small  northeast-southwest  trending  area  of  Gunflint  formation  rocks  exposed 
on  the  southeast  shore  of  Disappointment  Lake.  Here  the  sediments  have 
a  strike  corresponding  very  closely  to  the  trend  of  the  area  itself — that 
is,  northeast-southwest — and  they  dip  to  the  south.  In  rocks  of  simihir 
age  on  Gobbemichigamma  Lake  the  structure  is  a  little  bit  more  compli- 
cated. In  this  case  the  sediments  have  been  folded,  and  as  a  result  we 
now  find  them  forming  in  the  main  a  syncline  plunging  toward  the 
northwest,  but  with  a  subordinate  anticline  near  the  center  wliieli  has 
an  axis  plunging  to  the  southeast.     In  the  narrow  belt  extending  from 


UPPER  HUKONIAN.  877 

sec.  34,  T.  65  N.,  R.  5  W.,  eastward  to  the  gi-eat  cross  valley  in  sec.  27, 
T.  65  N.,  R  4  W.,  the  members  of  the  series  rest  upon  the  older  rocks 
and  unifoimly  dip  to  the  south.  The  regularity  of  this  dip  is,  however, 
interrupted  by  a  number  of  minor  flexures  whose  axes  plunge  south- 
southeast.  As  a  result,  the  amount  of  the  dip  varies  considerably,  ranging 
from  about  10°  to  65°  to  the  south,  the  higher  dips  occurring  invariably 
at  the  western  end  of  the  belt,  the  dips  becoming  flatter  as  the  belt  is 
followed  to  the  east.  Moreover,  these  dips  vary  rapidly  within  short  dis- 
tances. Likewise  the  strike,  although  in  general  following  the  trend  of 
the  belt,  is  found  to  vary  gradually  within  short  distances.  The  uniform 
dip  to  the  south  shows  the  very  simple  structure  which  prevails  in  this  belt, 
but  the  changes  in  angle  of  dip  and  in  strike  clearly  indicate  the  presence 
of  a  number  of  subordinate  rolls  in  these  monoclinal  southerly  dipping 
series  of  sediments.  The  gradual  diminution  in  the  angle  of  dip  as  the 
sediments  are  followed  to  the  east  corresponds  to  the  less  folded  condition 
of  these  sediments  in  this  part  of  the  area.  Attention  has  already  been 
called  to  the  areal  distribution  of  the  sediments  and  the  westward-trending 
tongue  of  sediments  occurring  in  sees.  21  and  22,  T.  65  N.,  R.  4  W.,  which 
is  good  evidence  of  an  infolded  syncline  of  these  sediments  at  this  place. 
The  dip  of  the  sediments  as  observed  upon  the  outcrops  in  this  area  gives 
further  proof  of  the  occurrence  of  this  syncline. 

In  general,  then,  the  sediments  have  a  uniform  dip  to  the  south,  with 
minor  irregulaiities,  these  irregularities  being  most  marked  in  the  western 
part  of  the  area  and  in  general  wherever  the  sediments  lie  against  the 
older  rocks.  Some  very  considerable  irregularities  have  been  noted  in  a 
few  cases  along  the  margins  of  certain  enormous  masses  of  dolerite  which 
occur  in  the  midst  of  the  sedimentary  area.  These  dolerites,  it  may  be 
stated  here,  are  intrusive  in  the  sediments,  and  this  fact  sufficiently  explains 
the  contorted  character  of  the  sediments  immediately  adjacent  to  them,  for 
this  contorted  chai'acter  is  confined  only  to  their  immediate  vicinity,  the 
uniform  low  southerly  dip  appearing  by  the  time  one  has  gone  some 
distance  from  such  contact  lines. 

PETROGRAPHIC  CHARACTEPS. 

The  Gunflint  iron-bearing  rocks  at  the  east  end  of  the  Vermilion  district 
correspond  stratigraphically  to,  and  are  indeed  the  eastern  continuation  of, 
the  iron-bearing  rocks  of  the  Biwabik  formation,  which  are  so  well  developed 


378  THE  VERMILION  IRON-BEARING  DISTRICT. 

and  of  sucli  enormous  economic  value  in  the  Mesabi  range.  Although 
stratigraphically  the  same  as  the  Biwabik,  the  rocks  at  the  eastern  end  of 
the  Vermihon  district,  constituting  the  Gunflint  formation,  have  been  in 
o-eneral  much  more  metamorphosed  than  the  Biwabik,  and  while  showing 
their  derivation  from  rocks  similar  to  those  constituting  the  Biwabik,  they 
are  now  petrographically  very  different  from  them. 

The  rocks  of  the  Biwabik  formation  have  been  described  by  the  Min- 
nesota Geological  Survey,  especially  by  N.  H.  and  H.  Y.  Winchell "  and 
J.  E.  Spurr,''  and  a  later  and  more  accurate  description  has  been  given  by 
Leith."     To  these  articles  the  reader  is  referred  for  details. 

.  The  following  brief  summary  made  by  Leith,  while  preparing  the 
report  on  the  Mesabi  district,  describes  the  petrographic  character  of  the 
rocks  typically  developed  in  that  district  and  may  aid  in  interpreting 
the  petrography  of  the  Gunflint  iron-bearing  rocks: 

The  Mesabi  iron  formation  rocks  are  mainly  ferruginous  chert,  but  contain 
also  iron  ore,  small  quantities  of  iron  and  calcium  carbonates,  thin  seam.s  of  slate  and 
paint  rock,  and,  finall}-,  certain  peculiar  green  rocks  containing  minute  dark-green 
o-ranules  resembling  an  indurated  greensand.  The  ferruginous  chert  and  the  iron 
ores  have  been  shown  to  develop  mainl}^  from  the  alteration  of  the  last-named  rock. 
The  original  green  granules  under  the  microscope  are  seen  to  lie  in  a  matrix  of  chert 
with  a  variety  of  textures.  They  have  round,  oval,  crescent  shaped,  gourd  shaped, 
or  more  irregular  forms;  their  color  varies  from  a  bright  green  through  a  shade  of 
yellowish  green  to  dark  brown;  under  crossed  nicols  a  tine  aggregate  polarization 
appears,  so  fine  that  the  substance  appears  practically  isotropic.  The  green  granules 
were  supposed  by  Spurr  to  be  true  glauconite,  but  later  work  by  the  United  States 
Geological  Survey**  shows  the  substance  to  be  essentially  a  hydrated  ferrous  silicate 
lacking  potash,  and  quite  different  in  composition  from  glauconite.  Moreover,  instead 
of  being  entirely  organic,  as  supposed  by  Spurr,  the  substance  of  the  green  granules 
is  supposed  to  be  the  result  of  chemical  developement  in  a  manner  analogous  to  the 
development  of  the  iron  carbonates  described  by  Van  Hise  for  the  other  districts  of 
the  Lake  Superior  region.''  The  shapes,  however,  may  be  due  to  filling,  replacement, 
or  accretion  about  minute  organic  bodies,  which  are  probably  commensui'ate  in 
variety  both  with  those  depositing  glauconite   and  with  those  giving  the  granule 

aGeol.  and  Nat.  Hist.  Survey  of  Minnesota,  Bull.  No.  6,  1891,  pp.  113-146. 
i>  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Bull.  No.  10,  1894,  p.  259. 

t-The  Mesabi  iron-bearing  district  of  Minnesota,  by  C.  K.  Leith:  Mon.  U.  S.  Geol.  Survey 
Vol.  XLIII,  1903,  pp.  101-159. 
''Ibid.,  p.  108. 
«  Twenty-first  Ann.  Kept.  U.  S.  Geol.  Survey,  Pt.  Ill,  1901,  jip.  32&-328. 


UPPER  HURONIAN.  379 

shapes  to  much  of  tho  CUnton  ore.  All  stages  of  the  alteration  of  this  green  ferrous 
silicate  rock  to  the  ferruginous  cherts  and  iron  ores  are  to  be  observed.  Scarcely  a 
slide  of  the  cherts  does  not  show  some  traces  of  the  granules.  The  alterations  have 
been  for  the  most  part  characteristic  of  surface  conditions  and  have  consisted  in  the 
decomposition  of  the  ferrous  silicate,  the  oxidation  of  the  ferrous  iron  to  the  hydrated 
hematite  form,  and  its  segregation  from  the  silica.  Where  metamorphosed  by  the 
Keweenawan  gabbro  the  alteration  of  the  granules  has  consisted  in  the  development 
of  a  variety  of  amphiboles,  including  actinolite,  grunerite,  cummingtonite,  and 
perhaps  others,  of  which  grunerite  is  the  most  abundant,  and  the  partial  oxidation  of 
the  feri'oiis  iron  to  the  magnetite  form. 

In  the  Grunflint  formation  the  rocks  very  commonly  have  structural 
characters  indicating  their  development  from  ferrous  silicate  granules  in 
the  manner  characteristic  of  the  metamorphism  by  the  gabbro — that  is, 
traces  of  the  granular  structures  still  remain;  but  the  characteristic  min- 
erals are  magnetite  and  the  amphiboles  resting  in  a  chert  matrix.  In 
addition  to  the  rocks  that  give  a  good  indication  of  the  kind  of  rock 
from  which  they  were  produced,  there  are  others  that  give  no  such  clue. 
They  are  without  characteristic  structural  features.  We  know  that  fer- 
ruginous carbonates  form  a  part  of  the  iron-bearing  formation,  and  it  is 
presumed  that  some  of  these  metamorphosed  products  have  been  derived 
from  such  carbonates.  It  is  impossible,  however,  to  give  any  quantitative 
estimate  of  the  relative  abundance  of  the  ferruginous  carbonate  and  ferrous 
silicate  rocks;  so  that  we  can  not  say  which  of  these  has  been  most  impor- 
tant in  furnishing  material  for  the  rocks  of  the  Grunflint  formation. 

In  general,  the  least  metamorphosed  of  the  Gunflint  rocks  are  thin 
bedded  and  consist  of  bands  of  nearly  pure  chert  alternating  with  cherty 
and  granular  quartzose  bands  containing  varying  percentages  of  iron  car- 
bonate, bands  of  jasper  and  magnetitic  chert,  and  others  consisting  of  quartz 
as  a  basis  with  actinolite  and  griinerite  crystals,  with  which  minerals  are 
always  associated  more  or  less  ferruginous  carbonate,  magnetite,  hematite, 
and  limouite.  A  description  of  the  least  altered  Gunflint  beds  lias  been 
given  by  Irving  and  Van  Hise  in  their  monograph  on  the  Penokee  iron- 
bearing  series."  The  cherty  ferruginous  carbonates  occur  in  better  develop- 
ment just  outside  of  the  Vermilion  district  in  Canadian  territory,  on  the 
north  shore  of  Gunflint  Lake,  than  in  the  Vermilion  district  proper.     The 

«Moii.  U.  S.  Geol.  Survey  Vol.  XIX,  1892,  pp.  260-268. 


380 


THE  VERMILION  IRON-BEARING  DISTRICT. 


following  analyses,  made  by  Mr.  Thomas  M.  Chatard,  of  the  United  States 
Geological  Survey,"  give  the   composition  of  some   of  these   carbonates. 

Analyses  of  iron-hearing  carbonates. 


VIII. 


IX. 


!~ilK'a 

Titanic  oxide 

Alumina 

Iron  sesquioxide  . . 

Iron  protoxide 

Manganese  oxide. . 

Calcium  oxide 

Magnesium  oxide  . 
Carbon  dioxide  . . . 
Phosphoric  acid . . . 

Iron  sulphide 

Water  at  110° 

Water  at  red  heat  . 

Total 


58.23 

Trace. 

.06 

5.01 

18.41 

.25 

.38 

9.59 

5.22 

.03 

.14 

.07 

2.01 


46.46 

Trace. 

.24 

.64 

26.28 

.21 

1.87 

3.10 

19.96 

.13 

.11 

.07 

1.15 


23.90 
None. 

.07 

.44 
10.65 

.28 

22.25 

8.52 

32.42 

Trace. 

.13 

.99 


99.40 


100.  2: 


99.  65 


VII  (specimen  10575) ,  iron  carbonate  from  Gunflint  beds  on  eastern  side  of  outlet  of  Gunflint  Lake 
on  international  boundary;  VIII  (specimen  10598),  from  same  beds,  but  from  northern  side  of  Gun- 
flint Lake;  IX  (specimen  10588),  ferriferous  carbonate  from  another  part  of  north  side  of  Gunflint  Lake. 

Under  the  microscope  most  of  the  above-mentioned  rocks  show  nothing 
of  especial  interest.  With  these  one  finds  chertv  ferruginous  rocks  which, 
when  examined  under  the  microscope,  are  of  interest,  since  thev  show  the 
relationship  of  these  rocks  to  the  less  altered  normal  rocks  of  the  iron  for- 
mation of  the  Mesabi  range,  concerning  which  a  brief  statement  was  made  a 
few  pages  back  (p.  378).  These  rocks  consist  of  rounded  areas  of  fine- 
o-rained  crvstalline  silica  and  limouite  and  rarelv  hematite — corresijonding- 
exactlv  in  shape  to  those  granules  which  have  been  mentioned  above — 
which  lie  in  a  groundmass  of  crystalline  silica  (see  PI.  XII,  A).  These 
areas  are  surrounded  by  a  border  df  limouite,  hematite,  or  these  oxides — 
most  commonly  limouite — are  more  or  less  uniformlv  distributed  through- 
out the  g'ranule  or  occasionallv  concentrated  at  tlie  center.  Within  tlie 
V)()rder  crystalline  silica  sometimes  predominates,  although  scattered  through 
it  tliere  is  more  or  less  limouite,  sometimes  actinolite  and  griuierite  and  a 
ferruginous  carbonate.  The  iron  oxide  has  frequently  a  definite  arrange- 
ment.     It  lias  accumulated  in  aggregates  at   the  centers  of  fibrous  quartz 


"  Men.  U.  S.  Geol.  Survey  Vol.  XIX,  1892,  pp.  191-192. 


PLATE    XIL 


381 


PLATE  XII. 

A,  Photomicrograph  showing  the  granules  in  tlie  Giinflint  formation.  These  granules,  which 
originally  consisted  of  a  green  to  brownish-green  hydrated  ferrous  silicate,  may,  after  alteration, 
consist  of  limonite,  hematite,  magnetite,  ferruginous  carbonate,  silica,  actinolite,  and  griinerite,  in 
various  combinations.  Limonite  and  silica  occur  very  commonly.  In  this  slide  the  granules  consist 
of  hematite  and  silica.  The  spherulitic  character  of  the  siliceous  matrix  is  well  shown  by  the  black 
crosses.     (Slide  29446,  21  diameters,  with  analyzer,  p.  380.) 

B,  Photomicrograpli  showing  the  details  in  a  limonite  silica  granule.  The  limonite  occurs  at 
the  centers  and  around  the  p-iripheries  of  small  spherulitic  or  granular  areas  of  silica.  In  the  upper 
right-hand  quadrant  an  area  with  well-defined  agate  structure  is  distinctly  shown.  (Slide  7004,  80 
diameters,  without  analyzer,  p.  383. ) 

382 


U.   S.   GEOLOGICAL   SURVEY 


MONOGRAPH    XLV       PL    XII 


A.  PHOTOMICROGRAPH    SHOWING    GRANULES    IN    GUNFLINT    FORMATION. 

B.  PHOTOMICROGRAPH    SHOWING    DETAILS   OF   GRANULES. 


UPPER  HURONIAN.  383 

spherulites,  and  around  each  one  of  these  spherulites  there  occurs  also  a 
fihn  or  thicker  layer  of  limonite,  with,  in  a  few  cases,  some  small  quantity  of 
hematite.  This  structure  is  interpreted  to  mean  that  the  ferrous  silicate 
originally  occupying-  these  areas  has  been  altered  into  its  constituents,  iron 
oxide  and  silica,  the  silica  forming  the  radiating  areas  above  mentioned,  and 
the  limonite  having  been  retained  either  at  the  centers  of  these  areas  or 
forced  outward  during  the  processes  of  crystallization,  so  as  to  form  a  ring 
now  surrounding  these  areas  (see  PI.  XII,  S).  That  a  large  part  of  the 
silica  of  the  granules  is  a  secondary  deposit  is  shown  b}^  the  fact  that  an 
imperfect  agate  structure  is  not  uncommon  (see  PI.  XII,  B).  A  similar 
agate  structure  also  occurs  between  the  large  rounded  granules  referred  to 
Projecting  from  the  sides  toward  the  centers  of  the  spaces  between  these 
granules  occur  also  segments  of  or  complete  quartz  spherulites.  This 
spherulitic  structure  showing  black  cross  is  reproduced  in  PI.  XII,  A. 

The  rocks  briefly  described  above  are  the  least  altered  forms  of  the 
rocks  of  the  iron-bearing  formation,  and  when  weathered  exhibit  on  the 
surface  a  brown  ferruginous  crust.  As  we  follow  these  rocks  westward  we 
find  that  they  change  somewhat,  passing  into  ferruginous  cherts  and  cherts 
which  have  been  more  or  less  completely  recrystallized  into  relatively  coarse- 
grained rocks  that  might  be  spoken  of  almost  as  quartzites — although  they 
are  not,  as  should  be  clearly  understood,  metamorphosed  clastic  sandstones — 
actinolite,  griinerite,  and  magnetite  rocks,  in  which  there  is  practically  no 
carbonate,  or  but  very  little.  These  rocks,  of  course,  vary  greatly  in  color, 
ranging  from  white  or  gray  to  brownish,  light  green,  dark  green,  and  prac- 
tically to  black,  the  color  depending  on  the  quantity  and  kind  of  the  minerals 
mentioned  which  are  present  in  them.  This  is  especially  true  of  those 
rocks  that  occur  in  the  narrow  belt  extending  from  a  short  distance  east  of 
Paulson's  mine  west  nearly  to  Gobbemichigamma  Lake.  Here  the  gabbro 
is  either  in  immediate  contact  with  or  but  a  short  distance  from  these  rocks. 
The  rocks  in  this  area  are  made  up  of  coarsely  crystalline  bands  of  quartz 
of  varying  width  in  alternation  with  coarsely  crystalline  bands  of  magnetite 
ore,  reported  to  range  from  1  inch  up  to  10  or  12  feet  in  thickness,  and 
bands  of  dark-green,  brown,  or  black  rocks,  which  consist  of  combinations 
of  quartz,  augite,  hypersthene,  hornblende,  olivine,  and  magnetite  as  the 
principal  minerals,  associated  occasionally  with  some  feiTuginous  carbonate, 
actinolite,  and  griinerite.  These  bands,  consisting  largely  of  ferromagnesian 
minerals,   vary  from  medium  grain  to   coarse  grain.      Occasionally  they 


384  THE  VERMILION  IRON-BEARING  DISTRICT. 

are  characterized  by  large  poikilitic  plates  of  hyperstheue  several  iiiches  in 
length,  which  show  bright  reflecting  faces.  Most  of  these  rocks  when 
examined  under  the  microscope  appear  as  granular  aggi-egates  of  the 
various  minerals  enumerated  and  give  no  clue  to  the  original  rock  from 
which  they  were  derived.  Some  of  them,  however,  still  contain  the 
rounded  areas  to  which  reference  has  already  been  repeatedly  made,  and 
show  conclusively  that  they  have  undergone  a  more  or  less  complete 
recrystallization.  In  these  the  areas  are  always  outlined  by  a  zone  of 
magnetite,  rarely  with  some  hematite.  In  some  cases  this  magnetite  occurs 
as  a  very  fine  dust;  in  others  the  magnetite  is  in  relatively  large  masses. 
Ordinarily  the  boundaries  between  these  areas  and  the  adjacent  quartz  of 
the  groundmass  are  sharply  marked  by  the  magnetite  zone.  "When  the 
areas  are  close  together  the  magnetite  border  of  tlie  one  unites  with  that  of 
the  ones  adjacent,  and  such  union  tends  to  destroy  the  regularit}'  of  the 
areas.  Indeed,  when  the  areas  are  closely  crowded  they  run  together  more 
or  less.  When,  in  addition  to  being  close  together,  the  interior  of  the  areas 
is  occupied  by  magnetite,  as  is  commonly  the  case,  the  resulting  rock  is 
composed  of  a  mass  of  magnetite  with  little  quartz  and  none  of  the  rounded 
granule  areas  are  visible.  In  man}^  of  the  areas  quartz  is  the  chief  con- 
stituent, in  relatively  coarse  grains.  Within  these  grains  occur  dust-like 
crystals  of  magnetite  which  are  accumulated  either  at  the  centers  of  the 
grains  or  just  within  their  peripheries.  Outside  of  the  areas  occurs  the 
matrix,  which  consists  now  of  coarsely  crystalline  quartz.  When  viewed 
between  crossed  nicols,  however,  it  is  seen  that  the  large  quartz  individuals 
of  which  the  matrix  is  composed  pass  across  the  boundary  and  extend  into 
these  areas. 

We  can  readily  see  how  such  a  rock  as  this  might  be  produced  from 
the  one  already  described  (p.  380),  in  which  essentially  the  same  conditions 
existed,  with  the  difference  that  the  rounded  areas  of  silica  and  limonite 
were  bounded  by  limonite,  and  that  the  quartz  was  in  fine  fibrous  spheru- 
litic  aggregates  with  limonite  at  the  centers  and  bounding  their  perij)heries 
(PI.  XII,  ^-1).  By  dehydration  of  the  limonite  there  would  be  produced 
hematite  or,  if  insufficient  oxygen  were  present,  as  appears  to  have  been  the 
case  throughout  this  region,  magnetite.  With  the  limonitic  rocks  there  is 
found  associated  ferruginous  carbonate,  which  also  contains  undoubtedly 
some  lime  and  magnesia.     Recrystallization  of  this  material  in  combination 


UPPER  HURONIAN. 


385 


with  the  h'on  and  sihca  of  the  adjacent  rock  might,  where  the  elements  were 
present  in  proper  proportion,  very  well  produce  the  actinolite,  griinerite, 
and  massive  hornblende  which  in  some  places  more  or  less  completely  fill 
out  these  rounded  areas. 

The  following-  partial  analyses  of  the  iron  ores  of  these  Gunflint  beds 
are  quoted  from  Wiuchell  and  Grant,  and  are  of  interest  as  showing  the  prac- 
tical absence  of  titanium  from  them."  In  this  respect  they  differ  very 
essentially  from  the  titaniferovis  magnetite  ores  which  form  a  part- of  the 
Duluth  gabbro  of  this  vicinity. 

Analyses  of  iron  ores  of  Gunflint  heds. 


Metallic  iron,  Fe 
Manganese,  Mn . 

Silica,  SiO^ 

Alumina,  AI2  O3 
Phosphorus,  P.. 
Titanium,  Ti 


I. 

II. 

.58.  40 

54.01 

4.92 

5.02 

8.22 

9.37 

0.52 

.0.07 

0.036 

0.032 

None. 

None. 

0.028 
Trace. 


62.05 


7.14 
0.113 


These  iron-bearing  rocks  were  observed  by  Chauvenet  in  his  recon- 
noissance  through  this  district  in  1884.''  They  were  also  studied  by 
Bayley  °  and  described  by  him  in  his  articles  upon  the  Duluth  gabbro  of 
Minnesota.  Chauvenet  apparently  considered  these  rocks  as  a  part 
of  the  gabbro.  W.  N.  Merriam  has  also  mapped  these  rocks  as  part 
of  the  gabbro,  and  Bayley  has  also  described  them  as  a  peculiar  periph- 
eral phase  of  the  gabbro.  H.  V.  Winchell,  of  the  Minnesota  survey, 
first  suggested  that  they  were  a  phase  of  the  Gunflint  formation  meta- 
morphosed by  the  gabbro.'*  This  is  beheved  to  be  the  correct  expla- 
nation of  the  orighi  of  these  peculiar  rocks.  This  explanation  was 
adopted  by  Spurr  and  also  by   Grant,  who  has    described   the  rocks  in 

a  Analyses  Nos.  I,  II,  III,  and  IV,  are  from  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Bull. 
No.  6,  1891,  p.  138.  No.  V  is  from  Geology  of  the  Mesabi  Iron  Range,  by  U.  S.  Grant:  Engineers' 
Year  Book,  University  of  Minnesota,  1898,  pp.  49-62.  Geol.  and  Nat.  Hist.  Survey  of  Minnesota, 
Final  Eept.,  Vol.  IV,  1899,  p.  480. 

("W.  M.  Chauvenet,  U.  S.  Geol.  Survey,  MS.  Notes. 

cThe  basic  massive  rocks  of  the  Lake  Superior  region,  by  W.  S.  Baylev:  Jour.  Geol.,  Vol.  I, 
1893,  pp.  433-716. 

''Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Twentieth  Ann.  Rapt.,  1893,  pp.  120-121,  134-136. 

MON  XLV — 03 25 


386  THE  VERMILION  IRON-BEARING  DISTRICT. 

some  detail  and  has  brought  forward  evidence  in  favor  of  the  hypoth- 
esis that  they  originated  by  metamorphic  action  of  the  gabbro  on  the 
Gunflint  formation.  Grant  bases  his  conclusions  as  to  their  mode  of 
origin  on  the  following  facts:  The  magnetite  in  the  Gunflint  beds  is,  as 
shown  by  the  analyses  quoted  above,  nontitaniferous,  whereas  that  of 
the  g'abbro  is  titaniferous ;  the  large  amount  of  quartz  in  these  beds 
could  not  possibly  be  derived  in  such  quantity  from  the  cr3"stallization 
of  the  gabbro  magma;  feldspar  is  absent,  whereas  it  is,  of  course,  an 
essential  constituent  of  the  gabbro  itself  A  further  fact,  which  should 
be  considered  as  evidence  against  the  view  that  these  iron-bearing  beds 
with  the  bands  of  ferromagnesian  minerals  are  a  contact  or  border 
facies  of  the  gabbro,  and  as  favoring  the  hypothesis  that  they  are  an 
exomorphic  contact  product  of  the  gabbro — the  explanation  which  is 
believed  to  be  the  correct  one — is  the  coarseness  of  the  beds  in  comparison 
with  the  recognizable  border  phases  of  the  gabbro  itself.  These  iron- 
bearing  rocks  range  from  medium-  to  coarse-grained  rocks.  In  general, 
they  are  coarser  than  the  border  phase  of  the  gabbro.  Such  a  condi- 
tion is  anomalous.  Ordinarily  the  contact  is  the  finer  grained  the  farther 
it  occurs  from  the  main  mass  of  the  igneous  rocks.  If  this  were  intei'preted 
as  a  contact  phase  of  the  gabbro,  here  we  would  have  next  to  the  main 
mass  of  gabbro  a  relatively  fine-  to  medium-grained  gabbro  and  then 
this  coarse-grained  facies,  which  in  places  is  made  up  of  bands  of 
coarse-grained  pure  quartz  and  the  other  bands  mentioned.  The  original 
rocks  from  which  the  iron-bearing  rocks,  and  eventually  these  rocks, 
were  derived,  judging  from  analogy  with  the  correlated  iron-bearing 
formation  of  the  Mesabi  district,  are  supposed  to  have  consisted  largely 
of  chert  with  a  hydrous  ferrous  silicate,  that  which  occurs  in  the  green 
granules,  with  which  is  associated  more  or  less  iron,  calcium,  and  mag- 
nesium carbonate.  From  rocks  of  this  composition  it  is  easy  to  see  that 
the  coarse-grained  rocks,  consisting  of  quartz,  magnetite,  olivine,  horn- 
blende, augite,  and  hypersthene,  might  have  been  derived  by  simple 
recrystallization,  without  presuming  any  transfer  of  material  from  the 
gabbro.  We  know  that  fai'ther  west  in  the  district,  where  the  gabbro  lies 
in  contact  with  the  Lower  Huronian  slates  and  conglomerates,  it  has 
metamorphosed  them  extremely,  producing  in  them  secondar}-  ferro- 
magnesian   silicates,    hypersthene,    hornblende,    biotite,    and    augite,    with 


UPPER  HURONIAN.  387 

varying  quantities  of  magnetite.  It  is  not  necessary,  therefore,  to  assume 
any  abnormal  conditions  other  than  the  contact  action  of  the  gabbro  on 
beds  having  the  proper  composition.  Complete  recrystallization  of  properly 
constituted  beds,  the  process  taking  place  slowly  and  extending  over  a  long- 
time, would  readily  account  for  the  existence  of  these  abnormal  Gunflint 
beds.  Since  we  consider  this  recrystallization  of  the  rocks  and  production 
of  magnetite,  etc.,  to  have  taken,  place  as  the  result  of  the  metaiuorphism 
produced  by  the  Duluth  gabbro,  it  is  evident  that  the  iron  in  the  rocks 
must  have  accumulated  prior  to  Keweenawan  time."  As  the  result  of  the 
metamorphism  the  rocks  were  so  changed  that  no  further  concentration 
of  iron  took  place,  and  consequently  we  find  these  deposits  in  this  part 
of  the  district  differing  both  in  petrographic  character  and  in  size  from  the 
great  deposits  of  the  western  Mesabi  or  Mesabi  proper,  whose  concentra- 
tion was  not  seriously  interfered  with  except  locally  during  Keweenawan 
times,  but  has  continued  right  on  up  to  the  present. 

RELATIONS   TO   OTHER  FORMATIONS. 

The  peculiar  Gunflint  formation,  found  at  the  base  of  the  Upper 
Huronian,  rests  upon  rocks  of  different  character  and  of  varying  age. 
These  range  from  the  granite  of  Saganaga  Lake  in  sees.  23  and  24,  T.  65  N., 
R.  4  W.,  through  the  Ely  greenstone  to  the  west  of  this  area,  and  then  up 
to  the  Ogishke  conglomerate  and  the  Knife  Lake  slates  still  farther  west. 
The  Duluth  gabbro  lies  against  and  upon  the  southern  edge  of  the  Gunflint 
formation. 

The  Gunflint  formation  of  the  Animikie  series  is  found  in  relationship 
with  the  Ely  greenstone  of  Archean  age,  at  one  especially  well-exposed 
place  in  the  north  side  of  the  cut  of  the  Duluth,  Port  Arthur  and  Western 
Railroad,  where  it  cuts  the  east  end  of  the  high  cliff  of  greenstone  on  the 
north  shore  of  Gunflint  Lake.  Here  the  iron  formation  is  well  banded, 
and  rests,  with  a  very  slight  dip  to  the  south,  on  the  crinkled  green  schists 
derived  from  the  Ely  greenstone.  At  one  place  about  1  foot  of  con- 
glomerate was  found  at  the  base  of  this  formation.  This  conglomerate  con- 
sists of  green  schist  and  quartz  pebbles,  and  above  this  comes  a  layer  of 
banded  white  chert  about  a  foot  thick  in  places,  and  somewhat  brecciated. 
The   iron   formation   proper   does   not   actually  appear  at  the    particular 

«Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Bull.  No.  10,  1894,  pp.  199,  358. 


388  THE  VERMILION  IRON-BEARING  DISTRICT. 

point  where  the  conglomerate  was  seen,  but  it  does  appear  a  few  paces  to 
the  east.  On  the  north  side  of  the  road,  in  sec.  30,  T.  65  X.,  R.  4  W., 
about  500  paces  east  6f  Fay  Lake,  there  were  found  several  trenches 
which  cut  through  the  iron  formation  and  showed  its  contact  with  the  EK- 
greenstone.  Here  it  seemed  to  rest  upon  this  greenstone  without  any 
intervening  conglomerate.  Still  farther  east,  at  the  west  end  of  the 
Duluth,  Port  Arthur  and  Western  Railroad,  just  west  of  Paulson's  camp, 
the  cut  has  exposed  the  Ely  greenstone  with  a  film  of  the  Gunflint  forma- 
tion lying  above  it.  At  this  place  no  well-marked  conglomerate  exists. 
The  greenstone  is  more  or  less  broken  up  and  some  of  the  iron  formation 
material  has  been  infiltrated  in  these  cracks,  so  that  on  the  surface  it  looks 
conglomeratic.  A  glance  at  the  maps  in  the  atlas  will  show  that  the  area 
just  mentioned,  in  which  the  two  exposures  of  greenstone  in  contact  with 
the  Gvmflint  formation  occur,  is  at  the  place  where  the  Ely  greenstone 
makes  its  greatest  bend  to  the  south. 

Relations  to  the  Lower  Huronian  series — Ogislike  conglomerate  and  Knife 
Lake  slates. — The  Ogishke  conglomerate  occurs  east  and  west  of  the  south- 
ward projecting  point  of  Ely  greenstone  in  sec.  30,  T.  65  N.,  R.  4  W.,  and 
sec.  25,  T.  65  N.,  R.  5  W.  It  is  very  thin  to  the  east,  and  in  fact  its  pres- 
ence has  been  detected  in  only  one  place,  as  the  result  of  an  examination 
of  the  dump  heaps  of  the  pits  northeast  of  Paulson's  mine,  in  the  NW.  \  of 
sec.  27,  T.  65  N.,  R.  4  W.  These  pits  are  just  north  of  the  ridge  of  Gun- 
flint  formation,  and  are  in  typical  bedded  rock.  This  bedded  rock  occurs  in 
the  upper  part  of  the  pit,  as  one  can  readily  see.  The  lower  part  of  the  pit 
is  now  filled  with  water,  but  some  rock  on  the  dump  and  that  forming  the  top 
part  of  the  dump,  presumably  material  last  taken  from  the  pit,  has  a  distinctly 
conglomeratic  appearance.  The  matrix  is,  liowever,  very  coarsely  crystal- 
line, and  the  supposed  pebbles  are  well  rounded.  The  kinds  of  rock  which 
constitute  the  pebbles  could  not  be  determined.  This  conglomerate  certainh- 
I'esembles  very  closely,  if  it  is  not  identical  with  the  Ogishke  conglomerate, 
wliiclL  occurs  farther  to  the  west.  There  is  a  bare  possibility  that  it  repre- 
sents a  conglomerate  at  the  base  of  tlie  Gunflint  formation  belonging  with 
the  Upper  Huronian  series,  but,  if  so,  it  could  not  be  discriminated  from 
the  Ogishke. 

West  of  the  southward-projecting  greenstone  point  above  mentioned 
the  Ogishke  conglomerate  appears  in  typical  development.     ^X  is  first  seen 


UPPER  HURONIAN.  389 

along  the  north  side  of  the  road  leading  to  Fay  Lake,  where  it  is  very  thin, 
but  to  the  west  it  increases  greatly  in  thickness,  until  it  is  found  covering 
an  area  having  a  width  north  and  south  of  nearly  a  mile  along  the  line 
between  sees.  26  and  27,  T  65  N.,  R.  5  W.  The  Gunflint  formation  has 
been  found  in  direct  contact  with  this  conglomerate  at  a  number  of  places, 
but  in  no  place  could  a  conglomerate  be  found  which  could  be  said  to  be  at 
the  base  of  the  Gunflint  formation  and  separable  from  the  Ogishke  con- 
glomerate. At  no  place  in  this  district  can  positive  e'V'idence  be  found  of 
the  relation  of  the  Gunflint  formation  to  the  underlying  Lower  Hui'onian 
series.  Where  these  rocks  are  in  contact  no  strike  or  dip  could  be  found  in 
the  conglomerates  below  the  iron-bearing  formation.  The  strike  and  dip 
can  be  determined  where  the  iron  formation  is  separated  from  the  Ogishke 
conglomerate  and  Knife  Lake  slates  by  an  area  within  which  there  are  no 
exposures.  In  these  cases  the  strikes  of  the  rocks  are  nearl}^  at  right 
angles  to  each  other,  and  there  is  a  great  discrepancy  in  dip.  This  evi- 
dence weighed  in  favor  of  an  unconformable  relationship.  The  evidence 
was  not  considered  conclusive,  for  in  view  of  the  close  folding  in  the 
district  the  possibility  was  recognized  thtit  conformable  rocks  may  have 
been  so  folded  that  with  lack  of  exposures  showing  the  actual  connection 
and  transition  they  may  appear  unconformable. 

Lidubitable  proof  of  the  unconformable  relationship  of  the  Gunflint 
iron-bearing  beds  to  the  Lower  Huronian  series  was  found  by  Leith  in  the 
Mesabi  district.".  There  the  conformable  series  of  rocks — Upper  Huronian — 
to  which  the  Gunflint  formation  belongs,  was  found  overlying  the  Lower 
Huronian  series  with  a  basal  conglomerate  between.  Thus  the  conclusion 
reached  from  the  study  of  the  imperfectly  exposed  beds  in  the  Vermilion 
district  was  confirmed. 

Belations  to  the  Keweenawan  (jDuluili)  gabbro. — In  all  cases  where  the 
Gunflint  foi-mation  is  exposed  in  the  Vermilion  district  it  is  found  that  the 
Duluth  gabbro  is  in  contact  with  the  series  on  its  southern  side.  It  is 
very  noticeable,  also,  that  in  all  cases  where  the  Gunflint  beds  are  in  con- 
tact with  this  gabbro  the  rocks  have  been  extremely  metamorphosed.  This 
metamorphism  is  most  noticeable  immediately  at  the  contact,  diminishing 
in  extent  as  the  distance  away  from  this  contact  increases.     These  facts 


f'The  Mesabi  iron-bearing  district  of  ilinnesota,  by  C.  K.  Leith:  Mon.  U.  S.  Geol.  Survey  Vol. 
XLIIl,  1903,  p.  180. 


390  THE  VERMILION  IRON-BEARING  DISTRICT. 

afford  indisputable  proof  that  the  gabbro  is   younger  than  the  Grunflint 
formation. 

Relations  to  basic  dikes. — The  Gunfliut  beds  have  been  cut  bv  dikes 
and  sills  of  basalt  similar  to  dikes  which  cut  the  Duluth  g-abbro. 

THICKNESS. 

The  Grunflint  formation  is  shown  on  the  map  in  the  atlas  as  feathering 
out  in  sec.  33,  T.  65  N.,  R.  5  W.,  near  the  end  of  Paiil  Lake.  East  of  this 
point  it  widens  very  much  and  reaches  its  maximum  width  in  sees.  23  and 
26,  T.  65  N.,  R.  4  W.  East  of  these  sections  it  again  narrows  down.  Where 
it  is  Avidest  the  beds  have  been  somewhat  crumpled  locally  by  great  intni- 
sive  sills.  Grant"  has  estimated  the  maximum  thickness  to  be  about  825 
feet,  calculated  on  an  average  dip  of  10°  S.,  but  states  that  this  estimate  is 
possibly  too  great. 

Irving  and  Van  Hise  estimated  the  thickness  of  the  iron-bearing  mem- 
bers in  the  Penokee  district  of  Wisconsin  and  Michigan  to  be  from  800  to 
1,000  feet. ''  This  seems  to  be,  however,  very  close  to  the  true  thickness 
for  the  Gunflint  iron -bearing  beds,  as  shown  by  comparison  with  correla- 
tive beds  in  other  districts.  Thus  the  Biwabik  formation  of  the  Mesabi 
district,  with  which  this  is  correlative,  has  been  estimated  by  Leith ''  to 
vary  from  200  to  2,000  feet  in  thickness,  and  to  have  an  averag-e  thickness 
of  about  1,000  feet. 

SECTION  II.— RO^TE   SLATE. 

The  sediments  constituting  this  formation  lie  immediately  above  the 
Gunflint  formation  and  have  been  called  the  Rove  slate  from  Rove  Lake, 
a  lake  sitviated  just  north  of  the  international  boundary  and  east  and  out- 
side of  the  Vermilion  district,  and  lying  in  a  large  area  underlain  by  these 
slates  in  typical  development.  Slates  occupying  the  same  stratigraphic 
position  and  possessing  the  same  general  characters  occur  in  the  Mesabi 
district  and  are  there  known  as  the  Virginia  formation. 

"Oeol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV,  1S99,  p.  471. 
''Mon.  U.  S.  Geol.  Survey  Vol.  XIX,  1892,  p.  189. 

«The  Mesabi  iron-bearing  district  of  Minnesota,  by  0.  K.  Leith:  Mon.  U.  S.  Geol.  Survey  Vol. 
XLIII,  1903,  p.  166. 


UPPER  HURONIAN.  391 

DISTRIBUTION,   EXPOSURES,   AND   TOPOGRAPHY. 

Distribution. — lu  the  Mesabi  district  south  and  west  of  the  Vermilion 
range  these  slates  cover  a  large  area.  The  Rove  slates,  which  are  found 
in  the  Vermilion  district,  represent  merely  a  portion  of  the  slates  that  cut 
across  the  east  end  of  the  district  and  that  are  not  cut  out  by  the  Duluth 
gabbro. 

The  westernmost  exposures  of  these  slates  in  the  Vermilion  district  are 
found  in  sec.  26,  T.  65  N.,  R.  4  W.  The  formation  underlies  a  very  narrow 
area  in  the  south-central  part  of  the.  above  section,  but  rapidly  widens  to 
the  east.  The  northern  boundary  of  the  slates  extends  northeastward,  and 
is  limited  by  the  Gunflint  formation  and  a  grea.t  dolerite  sill.  The  southern 
boundary  trends  east-southeast  and  the  Duluth  gabbro  everywhere  marks 
their  southern  extent.  At  the  eastern  limit  of  the  map  the  extreme  width 
of  the  Rove  slate  area  in  the  United  States  is  only  about  2  miles,  and  a 
great  deal  of  this  width  is  taken  up  by  intrusive  sills  of  dolerite,  which  very 
materially  reduce  the  areal  distribution  of  the  sediments.  Beyond  the 
limits  of  the  map  the  slates  have  an  enormous  development  in  Minnesota 
and  in  the  adjacent  portion  of  Canada.  They  may  be  seen  especiallv  well 
along'  the  Canadian  shore  of  Lake  Superior  and  on  the  islands  in  the  lake 
from  Pigeon  River  northeastward  to  Thunder  Cape. 

Exposures. — The  exposures  are  usually  very  good  along  the  lake  and 
wherever  steep  escarpments  occur,  which  is  usually  immediately  along  the 
lake  shores.  When  the  hills  stand  some  distance  back  of  the  lakes  it  is  not 
uncommonly  found  that  although  the  northern  slope  is  fairly  steep,  a  heavy 
talus  conceals  the  greater  portion  of  the  slates.  The  slates  rarely  show  any 
exposures  at  all,  or  but  very  poor  ones,  on.  the  gentle  southern  slope  of  the 
hills. 

Topography. — The  topography  is  that  which  is  usually  developed  in 
areas  of  monoclinal  dipping  rocks.  Ridges  have  been  formed  whose  trend 
correspoiids  approximately  to  the  strike  of  the  slates,  here  about  east  and 
west.  These  ridges  have  very  steep  escarpments  on  their  north  faces, 
where  the  rocks  have  been  cut  directly  across  the  dip,  and  very  gentle 
slopes  to  the  south  which  agree  in  general  with  the  dip.  The  depressions 
between  the  ridges  are  occupied  by  lakes,  or  if  not  by  lakes  then  by  low 
ground  with  a  stream  which  eventually  flows  into  a  lake.     Seen  in  profile 


392 


THE  VERMILION  IRON-BEARING  DISTRICT. 


the  ridges  and  intervening-  low  g-roiind  present  an  appearance  very  similar 

.      -  to  that  of  the  teeth  of  a  saw,  and  from  this  circumstance 

•£  they  are  sometimes  called  sawtooth  mountains.     Starting 

I  at  the  uorth  at  a  lake  one  ascends  a  steep  ridge  rising 

0  200  or  300  feet  above  the  lake  in  many  instances,  then 

1  descends  the  gentle  dip  slope  to  the  south,  which  leads 
I  down  to  a  second  depression  occupied  by  a  lake,  then 
Z  ascends  again  a  steep  northward-facing-  hill  with  gentle 
M  southerly  slope,  and  so  on.  Dolerite  sills  occur  interca- 
I  lated  in  these  Rove  slates  and  usually  cap  the  hills. 
I  Their  influence  on  the  topography  is  referred  to  later  on 
I  (p.  400).  The  topographic  character  of  that  portion  of 
a.  Minnesota  underlain  by  the  Rove  formation  can  be  seen 
i  in  PI.  XIII,  A,  and  fig.  21  in  the  text. 


STRUCTURE. 


^.2 


The  structure  of  the  slates  in  this  area  is  very 
simple.  Wherever  they  have  been  examined  the)''  are 
found  to  have  a  verj-  uniform  dip  of  from  5°  to  25°  to 
the  south-southeast.  They  evidently  form  a  part  of  the 
great  mpnoclinal  series  of  slates  which  are  known  as  far 
west  as  the  Mississippi  River  in  the  Mesabi  district,  and 
which  continue  directly  east,  cross  the  east  end  of  the 
Vermilion  district,  appear  on  the  north  shore  of  Lake 
Superior  north  of  Grand  Portage,  and  continue  thence 
eastward  around  Tlumder  Bay  and  northeastward  along 
the  sliore  of  Lake  Superior  for  some  distance.  As  indi- 
cated by  the  variation  in  dip — from  5°  to  25° — the 
monocline  of  slates  is  occasionally  interrupted  by  minor 
rolls,  which,  though  of  little  importance,  can  be  noted  by 
close  examination  of  almost  any  of  the  great  clift's  that 
give  good  exposures. 

PETROGRAPHIC  CHARACTERS. 

The  slates  form  the  bulk  of  the  Rove  formation,  but 
with  them  are  associated  graywackes,  some  quite  slaty, 
others  very  massive,  and  also  some  fairly  pure  quartzites. 


U.    S.   GEOLOGICAL   SURVEY 


MONOGRAPH    XLV      PL.    XIII 


SAWTOOTH    HILLS   OF    ROVE  SLATE   CAPPED    WITH    DOLERITE  SILLS.   AT   NORTHEAST   END   OF 
ROSE   LAKE,    INTERNATIONAL   BOUNDARY. 


Ji.     VIEW   ON    AN    ISLAND    IN    BURNTSIDE   LAKE,   SHOWING   GRANITE  OF    BURNTSIDE   LAKE   CUTTING 
AMPHIBOLE-SCHISTS— METAMORPHOSED    ELY   GREENSTONE. 


UPPER  HURONIAN.  393 

This  series  of  sediments  has  been  divided  by  Grant,"  of  the  Minnesota 
survey  into  a  "black  slate  member,"  with  a  "graywacke  slate  member" 
above  it.  In  our  work  no  attempt  has  been  made  to  discriminate  between 
these  two  petrographic  facies  of  the  Eove  formation.  They  are  not 
separable  by  any  time  interval,  but  represent  merely  slight  changes  in  the 
conditions  of  deposition.  Macroscopically  they  are  very  fine-grained  black 
carbonaceous  slates,  grading  up  into  dark-gray  graywackes  of  medium 
grain,  with  occasional  bauds  of  material  almost  sufficiently  pure  to  be  called 
quartzite.  In  no  case  were  any  conglomerates,  even  fine-grained  ones, 
found  associated  with  these.  The  slates  are  unquestionably  the  predomi- 
nant kind  of  rock  in  the  Vermilion  district.  They  are  commonly  very 
fissile,  although  in  places  these  carbonaceous  rocks  are  fairly  massive. 

Microscopic  characters. — The  Rove  sediments  are  composed  of  angular 
quartz  and  feldspar  grains  in  a  dark  cement.  In  some  cases  the  character 
of  this  cement  can  be  partly  seen,  and  one  can  then  recognize  shreds  of 
biotite  and  chlorite.  Between  these  is  a  very  fine-grained  dark  material 
which  is  presumed  to  consist  of  nnnute  dust  particles  of  quartz  and  feld- 
spar and  ferruginous  and  carbonaceous  material.  Many  of  these  rocks 
are  so  well  crystallized  that  they  may  almost  be  called  phyllites.  In  these 
crystalline  rocks  the  matei'ial  between  the  grains,  jjrobably  formed  from 
the  decomposition  of  the  fine  matrix  above  referred  to,  consists  of  flakes 
of  biotite  and  chlorite,  with  quartz  and  ferruginous  matter. 

CONTACT  METAMORPHISM   OF  THE  ROVE  SLATE. 

The  Rove  slate,  as  has  already  been  stated,  is  in  contact  on  its 
southern  border  with  the  Dulutli  gabbro.  At  numerous  places  within 
the  formation  there  are  gi-eat  intrusive  sills  which  are  considered  to  be 
offshoots  from  the  Duluth  gabbro.  The  reasons  for  this  -sdew  will  be  given 
in  a  later  chapter.  The  gabbro  and  the  sills  have  liad  a  slight  contact 
effect  upon  the  slates  adjacent  to  them.  Actual  contacts  of  the  sills  with 
the  slates  in  this  district  were  not  seen,  but  a  number  of  contacts  of  similar 
sills  on  similar  slates  were  seen  along  the  Lake  Superior  shore  in  the 
Thunder  Bay  district  of  Canada,  and  in  all  such  cases  the  slates  merely 
showed  a  slight  induration.     Outside  of  the  district,  as,  for  instance,  on  Pigeon 


aGeol.  and  Nat.  Hist.  Survey  of  Minnesota,  Twenty-second  Ann.  Kept.,  1893,  p.  74;  Final  Kept., 
Vol.  IV,  1899,  p.  470. 


394  THE  VERMILION  IRON-BEARING  DISTRICT. 

Point,  Minnesota,  certain  gabbroic  intrusives  are  known  to  have  had  a  very 
far-reaching  contact  eflPect  on  these  sediments."  Along-  the  southern  and 
southeastern  sliores  of  Loon  Lake  were  collected  several  specimens  of 
sediments  which  were  near,  although  not  in  actual  contact  with,  the  sills. 
One  of  these  specimens  shows  a  spotted  character  and  is  a  spilosite  such 
as  is  fairly  common  in  sediments  near  the  contact  with  the  great  mass  of 
gabbro  and  occurs  also  in  other  districts  near  great  dolerite  dikes.  This 
spilosite  contains  a  large  amount  of  chlorite  in  clumps  embedded  in  a 
matrix  of  quartz  and  presumably  some  feldspar,  and  forms  the  microscopic 
spots.  In  the  Mesabi  range  some  of  the  slates  near  the  gabbro  contact 
show  clearly  recognizable  cordierite,  forming  the  white  spots,  and  the  slates 
have  been  metamorphosed  to  a  cordierite-hornstone.*  In  general  the  slate 
adjacent  to  these  sills  in  the  Vermilion  district  shows  its  normal  chai-acters 
with  at  most  a  little  metamorphism  due  to  cementation. 

A  contact  of  the  gabbro  with  the  Rove  formation  at  a  point  at  the 
southwest  end  of  Loon  Lake  was  examined.  This  contact  is  of  the  gabbro 
on  the  "graywacke  slate  member"  of  Grant.  The  sediments  at  the  top 
of  the  section  were  within  about  3  feet  of  the  gabbro.  This  is  as  near  as 
we  found  the  sediments  to  the  gabbro.  Here  the  rocks  are  interbanded 
slates  and  graywackes  which  were  quite  crystalline  and  hard.  Microscopic 
examination  of  them  shows  that  the  gabbro  had  effected  a  partial  recrys- 
tallization  of  the  sediments  and  discloses  in  the  sediments  a  large  amount  of 
secondary  biotite  and  muscovite.  Both  of  these  occur  in  relatively  large 
porph-sTitic  plates  inclosing  grains  of  the  other  materials  constituting-  the 
slate,  recognizable  quartz,  and  ferruginous  material.  As  the  rocks  are 
studied,  as  we  go  down  the  slope,  they  are  seen  to  be  less  indurated,  until 
near  the  bottom  of  the  section  at  the  water's  edge,  about  50  feet  below  the 
gabbro,  the  sediments  do  not  appear  essentially  different  from  the  ordinary 
rocks  of  this  character  and  of  this  age.  It  is  clear  from  this  that  the  effect 
of  the  gabbro  has  not  been  felt  at  a  very  great  distance  from  the  actual 
plane  of  contact  with  the  sediments. 

«0n  some  peculiarly  spotted  rocks  from  Pigeon  Point,  Minnesota,  by  W.  S.  Bayley:  Am.  Jour. 
Sci.,  M  series,  Vol.  XXXV,  1888,  pp.  388-.393.  Abstract,  Nature,  Vol.  XXXVII,  1888.  p.  91  (.5  lines). 
Rocks  on  Pigeon  Point,  Minnesota,  and  their  contact  phenomena:  Bull.  V.  S.  Geol.  Survey  Xo.  109, 
1893,  pp.  121. 

("The  Mesabi  iron-bearing  district  of  iliunesuta,  by  C.  K.  Leith:  Mon.  U.  S.  Geol.  Survey  Vol. 
XLIII,  190.3,  pp.  171-172. 


UPPER  HURONIAN.  395 

RELATIONS    TO   ADJACENT   FORMATIONS. 

The  relations  of  the  Rove  slates  to  the  other  formations  of  the  district 
are  easily  deteriiiined.  The  oldest  rock  with  which  they  are  in  contact  is 
that  which  has  been  described  as  the  Gunflint  formation.  The  slates  are  a 
conformable  series  of  sediments  overlying-  this  formation,  and  consequently 
younger  than  it.  In  previous  pages  the  relations  of  the  Gunflint  formation 
to  the  other  older  rocks  of  the  district  have  been  described,  and  it  is  not 
necessary  to  add  anything-  to  the  statement  concerning  the  age  of  the  Rove 
slates  other  than  that  they  are  young-er  than  all  of  the  rocks  below  the 
Gunflint  formation. 

Relations  to  Keiveenaivan  dolerite. — In  places  the  Rove  slates  are  found 
in  contact  with  great  masses  of  dolerite.  Near  the  contact  the  sediments 
are  found  to  be  harder  than  elsewhere,  and  in  some  places  to  have  had 
produced  in  them  minerals  which  are  evidently  of  secondary  origin,  corre- 
sponding to  the  products  of  contact  metamorphism,  which  have  been 
studied  in  other  districts.  This  induration  is  undoubtedly  due  to  the 
metamorphic  action  of  the  dolerite.  This  alone  is  proof  of  the  fact  that  the 
dolerite  is  younger  than  the  sediments.  In  addition  to  this  proof,  however, 
we  have  the  further  evidence  of  the  contortion  of  the  slates,  which  has  been 
noticed  in  a  number  of  places  where  the  beds  were  in  contact  with  the 
dolerites,  having  been  intruded  by  them.  Moreover,  the  dolerites  them- 
selves are  much  finer  grained  at  the  edge  than  elsewhere.  These  three 
facts — the  fine-grained  character  of  the  edges  of  the  dolerite  masses,  the 
induration  of  the  slates  along  this  contact,  and  the  contortion  of  the  beds — 
form  indubitable  evidence  that  the  dolerites  are  younger  than  and  intrusive 
in  the  slates. 

Relations  to  Keweenawan  (Bulutli)  gahhro. — The  only  place  where  a  good 
contact  between  the  gabbro  and  the  slates  was  observed  is  that  mentioned 
above,  on  the  southwest  side  of  Loon  Lake.  Here  the  gabbro  overlies  the 
slates,  and  produced  considerable  changes  as  the  result  of  its  contact  with 
them.  The  superposition  of  the  gabbro  and  the  contact  zone  in  the  slates 
afford  conclusive  proof  of  the  relative  ages  of  the  two,  the  Duluth  gabbro 
being  very  clearly  younger  than  the  Rove  slate. 


396  THE  VERMILION  IRON-BEARING  DISTRICT. 

AGE. 

Fi'om  the  foregoing  statements  we  see  that  the  Rove  slates  and 
graywackes  form  the  youngest  member  of  the  Animikie  series  in  the 
Vermihon  district.  The  only  rocks  younger  than  it  are  the  dolerite  sills, 
the  Duluth  gabbro,  and  the  occasional  basic  and  acid  dikes  which  cut 
through  the  gabbro. 

THICKNESS. 

Only  a  very  small  portion  of  the  sediments  which  constitute  the  Rove 
slates  in  the  Lake  Su])erior  region  are  represented  in  the  Vermilion  disti-ict. 
As  has  been  shown  by  the  distribution,  only  the  apex  of  a  V  which  rapidly 
widens  to  the  east  is  there  present.  The  gabbro  of  Keweenawan  age  comes 
in  from  the  south  and  swings  up  northwestward,  cutting  across  the  east- 
west  striking  slates,  and  ^^roducing  the  V  above  referred  to.  In  the  Ver- 
milion district,  then,  the  Rove  slates  vary  from  a  minimum,  at  the  point  of 
the  V,  up  to  a  maximum  for  that  district  which  attains  a  considerable  thick- 
ness. No  attempt  to  measure  the  maximum  thickness  for  this  district  has, 
however,  been  made,  as  it  would  give  merely  the  thickness  of  a  portion  of 
the  slate  formation,  and  not  that  of  the  formation  as  a  whole.  The  latest 
estimate  for  that  part  of  the  slates  present  in  the  Vermilion  district  is 
that  made  by  the  Geological  and  Natural  History  Survey  of  Minnesota. 
According  to  this,  the  "Graywacke  Slate  Member"  has  a  thickness  of  l,(i50 
feet,  the  "Black  Slate  Member"  a  thickness  of  950  feet,  and  the  sills  intruded 
in  these  rocks  a  thickness  of  about  250  feet.  This  gives  a  total  thickness  for 
the  sediments  of  the  Rove  formation  exposed  in  this  district  of  2,600  feet. 
No  statement  is  made  as  to  the  section  on  which  the  estimate  of  this  thick- 
ness was  based,  but  it  was  presumably  between  Gunflint  and  North  and 
South  lakes,  just  east  of  the  limits  oi  the  area  shown  on  the  accompanying 
map  of  the  Vermilion  district,  PL  II.  The  formation  has  been  studied  at 
various  places  by  a  number  of  geologists,  and  varying  estimates  have  been 
made  of  its  total  thickness.  According  to  estimates  made  by  Irving,"  the 
Animikie  series  of  slates  corresponding  to  the  Rove  slates  of  the  report  has 
a  thickness  of  10,000  feet.    Ingall  has  estimated  this  thickness  at  12,000  feet.*" 

In  1892  Irving  and  Van  Hise''  gave  an  estimate  of  11,000  feet  as  the 
maximum  thickness  of  the  Animikie  slates  in  the  Penokee  disti'ict. 


«Mon.  U.  S.  (ieol.  Survey  Vol.  V,  1883,  p.  380. 

''Geol.  and  Nut.  Hint.  Survey  of  Canadii,  Ann.  Kept,  for  ISSS,  II,  p.  2(). 

'Mon.  U.  S.  Geol.  Survey  Vol.  XIX,  1892,  p.  299. 


CHAPTER  VI. 

THE  KEWEENAWAN. 

IKTRODUCTION. 

The  only  rocks  of  Keweenawan  age  in  the  Vermilion  district  are 
gabbros  which  form  a  part  of  the  Duluth  gabbro  mass  of  northeastern 
Minnesota,  certain  great  basic  sills  to  which  the  name  Logan  sills  has  been 
given,  and  some  few  basic  and  acid  dikes  which  cut  all  of  the  rocks  of  the 
district,  including  the  aforementioned  gabbros  and  Logan  sills.  As  a 
result  of  the  studies  reported  in  this  monograph,  it  has  been  determined 
that  stratigraphically  the  Duluth  gabbro  and  the  Logan  sills  belong  together, 
cilthough  they  show  slight  differences  in  lithologic  character.  These 
differences  are  due  essentially  to  variations  in  the  conditions  of  consoli- 
dation. Since  these  two  rocks  belong  together,  they  will  be  described 
under  the  same  section  in  the  following  pages.  A  second  section  will  be 
devoted  to  a  brief  mention  of  the  basic  and  acid  dikes,  which  are  the 
youngest  rocks  of  the  Vermilion  district,  excluding  always  the  Pleistocene 
glacial-drift  deposits. 

SBCTIOX  I.— DULUTH  OABBRO  ANT>  LOGAN  SILLS. 

Refei-ences  to  the  OTeat  o-abbro  mass  of  northeastern  Minnesota  are 
common  in  the  geologic  literature  of  the  Lake  Superior  region.  The  name 
Duluth  is  given  to  this  gabbro  since  it  is  so  well  developed  near  tlie  city  of 
that  name.  This  rock  is  conspicuously  developed  on  the  north  shore  of 
Lake  Superior,  where  it  forms  a  prominent  part  of  the  Keweenawan  series 
of  northeastern  Minnesota,  underlying  several  hundreds  of  square  miles.  It 
is  also  well  known  upon  the  south  shore  in  the  Keweenawan  district  of 
Wisconsin. 

North  of  the  Duluth  gabbro,  and  extending  all  around  the  north  shore 
of  Lake  Superior  as  far  as  the  Slate  Islands  of  the  northeast  shore  of  the 
lake,  it  has  been  found  by  the  various  geologists  who  have  worked  in  this 

397 


398  THE  VERMILION  IRON-BEARING  DISTRICT. 

territory,  beginning  with  Logan,"  that  the  sedimentary  rocks  of  this  region, 
slates,  quartzites,  and  graywackes,  have  intercalated  in  them  at  various 
horizons  sheets  of  basic  igneous  rock  ranging  in  thickness  from  1  to  100 
feet.  These  vary  in  character  from  distinctly  gabbroic  rocks  in  the 
centers  of  the  large  masses  through  all  gradations  of  finer-grained  granular 
and  porphyritic  rocks  to  the  very  fine-grained  basaltic  phases  which  form 
the  thin  sheets  and  occur  as  well-formed  selvages  of  many  of  the  thicker 
sheets.  These  are  the  intrusive  sheets  which  have  been  called  the  Los'an 
sills  by  Lawson,*  in  recognition  of  the  geological  work  done  by  that 
pioneer  of  investigation  in  this  field. 

DISTRIBUTION.   EXPOSURES,   AND   TOPOGRAPHY. 

Distribution. — The  Duluth  gabbro  forms  the  southern  boundaiy  of  the 
pre-Keweenawan  rocks  throughout  the  greater  portion  of  the  Vermilion 
district.  The  westernmost  point  at  which  the  Duluth  gabbro  touches  the 
district  is  in  sees.  26  and  3.5,  T.  63  N.,  R.  10  W.,  and  section  3,  T.  62 
N.,  R.  10  W.  From  these  sections  on  along  the  Kawishiwi  River  the  Duluth 
gabbro  swings  off  to  the  northeast  with  a  broad  sweep,  extending  just 
within  the  area  mapped  as  far  east  as  the  vicinity  of  Paulson's  mine,  in 
sec.  28,  T.  65  N.,  R.  4  W.  From  this  place  its  edge  trends  to  the  southeast, 
passing  beyond  the  limits  of  the  area  mapped  toward  Lake  Superior. 
A  couple  of  small  isolated  outliers  have  been  found  north  of  Grobbemichi- 
gamma  Lake.  The  southernmost  one  is  only  a  quarter  of  a  mile  from  the 
northern  edge  of  the  main  mass  of  the  gabbro,  northwest  of  Paulson 
Lake,  and  the  other  is  about  three-fourths  of  a  mile  from  the  nearest  point 
on  the  edge  of  the  gabbro  and  lies  in  the  NW.  4  sec.  29,  and  NE.  -^  sec. 
30,  T.  65  N.,  R.  5  W. 

The  sills  lie  well  within  the  district  to  the  north  of  the  edge  of  the  gabbro 
mass,  varying  in  distance  from  this  edge.  The  first  exposure  of  such  a  sill 
was  noticed  on  the  southwest  side  of  Gobbemichigamma  Lake,  but  this  can 
not  be  traced  far.  The  next  one  was  seen  near  Bingoshick  Lake.  This  sill 
has  been  followed  to  the  east  for  several  miles  to  a  point  east  of  Paulson's 
mine,  having  throughout  this  distance  an  almost  continuous  outcrop.  Par- 
allel to  this  sill  sevei"al  small  and  relatively  unimportant  sills  have  been 


"Geological  Survey  of  Canada,  1846-47,  p.  13. 

''The  lacolitic  .sills  of  the  northwest  coa.st  of  Lake  Superior,  r)y  .-\.  ('.  LawHon:  (ieol.  ami  Xat. 
Hist.  Survey  of  Minnesota,  Bull.  No.  8,  pt.  2,  189:i,  jip.  48. 


THE  KEWEENAWAN.  399 

observed.  Beyond  Paulson's  mine  the  Upper  Huronian  sediments  begin  to 
widen,  rapidly  increasing  in  width  as  they  are  followed  to  the  east,  as 
alread}^  described.  Corresponding  with  this  widening,  we  find  an  increas- 
ing number  of  sills  having  in  general  a  trend  east  and  west  and  lying 
approximately  parallel  to  each  other.  During  several  trips  to  Gimfllnt 
Lake  and  to  the  country  to  the  south  a  number  of  these  sills  were  followed 
along  their  strike  for  short  distances  and  were  also  crossed  at  right  angles 
to  the  strike.  Their  relations  to  the  sediments  were  thus  clearly  seen. 
No  attempt  was  made  to  trace  out  the  individual  sills.  This  work  has 
been  done  in  previous  years  by  Chauvenet"  and  Merriam,''  of  the  United 
States  Geological  Survey,  and  in  more  recent  years  by  U.  S.  Grant,  °  of  the 
Minnesota  Survey. 

The  data  for  the  distribution  of  the  sills  which  are  shown  on  the  accom- 
panying map  have  been  taken  chiefly  from  the  reports  of  these  men. 

Exposures. — Throughout  the  area  underlain  by  the  gabbro,  as  well  as 
the  sills,  exposures  are  very  numerous  and  usually  of  large  size,  affording 
excellent  opportunities  for  the  study  of  the  characters  of  these  rocks,  their 
variations  in  grain,  and  also  their  relations  to  the  adjacent  sediments. 

Topography. — The  line  of  contact  between  the  gabbro  and  the  older 
rocks  adjacent  to  it  is  fairly  well  marked  by  a  slight  topographic  break.  The 
gabbro  normally  has  a  steep  north  face  sometimes  showing  an  escarpment  of 
varying  height.  It  is  never  very  high,  but  is  considerably  higher  than  any 
topographic  features  in  the  area  north  of  it  for  some  distance.  The  contact 
is  frequently  marked  by  a  lake  or  a  stream.  This  difference  between  the 
topography  of  the  gabbro  area  and  that  to  the  north  exists  at  the  immediate 
contact,  but  examining  contrasting  areas  as  a  whole  we  find  that  in  general 
the  gabbro  area  is  lower  than  that  underlain  by  the  older  formations  to  the 
north.  Locally  the  gabbro  area  has  been  reduced  almost  to  base  level.  In 
fact,  this  area  may  be  described  as  very  nearly  a  plain,  but  one  with  minor 
but  pronounced  irregularities.  The  uniformity  of  the  surface  is  due  in  g-reat 
part  to  the  homogeneous  character  of  the  gabbro  mass,  which  has  caused  it 
to  be  about  equally  affected  by  the  various  agents  which  have  attacked  it. 
The  minor  pronounced  irregularities  are  usually  found  to  be  due  to  erosion, 
which  has  been  controlled  very  frequently  by  the  joints  of  the  gabbro,  and 

«  W.  M.  Chauvenet,  XJ.  S.  Geol.  Survey,  manuscript  notes. 
i  Mon.  U.  S.  Geol.  Survey  Vol.  XIX,  1892,  PL  XXXVII. 
cGeol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Eept,,  Vol.  IV,  1899,  pp.  487^88. 


400 


THE  VERMILION  IRON- BEARING  DISTRICT. 


to  differences  in  composition  where  such  exist.  For  example,  the  anorthosite 
masses  usually  stand  out  conspicuously  from  the  surrounding  more  basic 
and  less  resistant  portions  of  the  gabbro. 

The  lakes  of  tlie  gabbro  area  are,  as  a  rule,  shallow,  and  they  are  also 
very  irregular,  and  can  not  be  said  to  possess  uniformit}-  of  long  extension 
in  any  one  direction,  as  is  so  markedly  the  case  in  the  lakes  of  the  other 


Fia.  22.— Set'tiun  through  the  Eove  shitcs.  with  intercaUited  Logan  sills  south  of  Gunflint  Lake. 

portions  of  the  Vermilion  district.  On  the  contrary,  they  spread  out  in  all 
directions,  sending  off  luimerous  bays,  some  of  which  a)-e  very  long  and 
narrow,  and  all  very  irregular  in  shape. 

The  Logan  sills  exercise  a  very  material  influence  upon  the  topography 
of  that  portion  of  the  district  north  of  the  gabbro  in  which  the}'  occur.  It 
will  be  recalled  that  the  Upper  Huronian  (Animikie)  sediments  in  this  vicin- 
ity have  a  iiionoclinal  dip  to  the  south.    The  sills  have  been  injected  essen- 


THE  KEWEENAWAN.  401 

tially  parallel  to  the  bedding-  of  the  sediments,  although  occasionally  they 
are  found  cutting  across  the  beds  at  low  angles.  Erosion  has  been  most 
active  in  this  portion  of  the  district  in  a  direction  parallel  to  the  strike  of 
the  beds,  and  consequently  most  of  the  large  valleys  trend  in  agreement 
with  these,  approximately  east  and  west.  The  resistant  sills  now  form  the 
caps  of  the  ridges,  the  slates  having  been  removed  down  to  the  sills.  The 
massive  rock  forming  the  sills  breaks  off  along  the  joint  planes,  and  this 
breaking  results  in  forming  perpendicular  cliffs,  below  the  foot  of  which 
talus  from  the  sills  and  from  the  easily  weathering  Rove  slates  give  a 
gentle  slope.  These  sills  are  sometimes  very  nearly  concealed  by  the 
accumulated  talus  deiived  from  them. 

The  efiFects  of  erosion  have  produced  a  series  of  hills  with  ver}^  nearly 
vertical  north  escarpments,  and  a  gentle  slope  from  the  crest  of  the  hills  to 
the  south.  This  slope  corresponds  very  closely  to  the  dips  of  the  Rove 
slates  and  the  upper  surface  of  the  dolerite  sills.  Fig.  22  shows  a  some- 
what idealized  section  through  the  ridge  on  the  south  side  of  Gunflint 
Lake,  taken  from  W.  M.  Chauvenet's  manuscript  notes. 

PETROGRAPHIC  CHARACTERS   OF  THE  GABBRO. 

Macroscopic  characters. — It  is  not  the  purpose  of  this  report  to  consider 
in  detail  other  rocks  than  those  of  pre-Keweenawan  age  which  make  up 
the  Vermilion  district  in  its  strict  sense.  In  order,  however,  to  give  a 
complete  description  of  the  area  shown  on  the  maps  in  the  accompanying 
atlas,  it  is  essential  to  consider  at  least  briefly  tlie  Dulutli  gabbro.  Speci- 
mens have  been  taken  here  and  there  along  its  margin,  and  several  trips 
have  been  made  well  down  into  the  gabbro,  during-  which  specimens  were 
collected  of  the  varieties  seen  and  observations  made  on  their  relations. 
Tlie  following  brief  description  of  the  gabbro  is  the  result  chiefly  of  the  study 
of  these  specimens.  No  attempt  has  been  made  to  obtain  specimens  from  all 
parts  of  the  gabbro,  and  consequently  numerous  facies  which  would  be  seen 
only  after  very  detailed  studies  of  the  gabbro  have,  of  course,  not  been 
found.  For  more  detailed  descriptions  of  this  gabbro  the  reader  is  referred 
to  the  reports  of  the  Minnesota  survey,  especially  to  the  articles  by 
Elftman",  Grant,''  and  Winchell,"  and  to  the  petrographic  study  of  the  gabbro 

aAm.  Geologist,  Vol.  XXII,  1898,  pp.  131-149. 

»Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV,  1899,  p.  476  et  seq.;  Twenty- 
third  Ann.  Kept.,  1894,  pp.  224-230. 

cAm.  Geologist,  Vol.  XXVI,  1900,  pp.  151-188,  197-245,  261-306,  348-388. 

MON  XLV — 03 26 


402  THE  VERMILION  IRON-BEARING  DISTRICT. 

by  Bayle}'."  The  gabbro,  as  a  rule,  was  found  to  be  a  medium-  to  coarse- 
grained rock,  with  essentially  the  same  granular  texture  throughout. 
However,  near  the  contact  of  the  gabbro  with  the  other  rocks  to  the 
north  of  it,  it  is  found  to  grow  much  finer  grained.  This  gradation 
is  rapid.  Thus  the  gradation  from  a  medium  fine-grained  to  a  normal 
coarse-grained  rock  was  completed  within  a  distance  of  about  10  paces. 
Occasionally  large  areas  of  the  fine-grained  phase  of  rock  which  has  been 
called  granulitic  gabbro  occur  in  the  midst  of  the  main  gabbro  mass.  This 
fine-grained  material  is  found  to  have  very  sharp  contacts'*  with  the  coarser- 
grained  gabbro,  and  small  areas  of  this  fine-grained  material  are  also 
included  in  rounded  as  well  as  irregular  masses  within  a  coarse-grained 
gabbro,  possibly  indicating  that  there  is  a  slight  diff"erence  in  age  between 
the  two.  This  fine-grained  gabbro  at  the  point  referred  to  has  a  remarkable 
horizontal  jointing,  as  the  result  of  which  it  looks  at  a  short  distance  like  a 
bedded  rock  in  layers  of  from  2  to  6  inches  thick.  It  also  has  a  sheeted 
structure  striking  in  a  general  way  east  and  west  and  dipping  to  the  south. 
This  structure  is  brought  out  by  the  differential  weathering.  In  some 
cases  also  the  gabbro  has  a  very  distinctly  banded  structure,  as  has  been 
described  by  Grant."  He  describes  an  exposure  near  south  end  of  Bald 
Eagle  Lake  as  having  a  gneissic  structure  which  is  practically  vertical  and 
runs  N.  15°  W.,  making  the  rock  break  more  readily  in  this  direction  than 
in  any  other.  "In  some  places  the  gabbro  lies  in  horizontal  beds  from 
2  to  4  inches  thick.  The  rock  seems  to  be  almost  entirely  composed  of  a 
feldspar  (probably  labradorite)  and  a  mineral  which  is  probably  olivine; 
this,  when  not  decayed,  is  of  a  yellowish  green  color."  Microscopic  studies 
show  this  mineral  to  be  olivine.  This  occuiTence  is  very  similar  to  the 
banded  faces  of  the  gabbro  which  may  be  seen  upon  the  St.  Paul  and 
Duluth  Railroad  between  Short  Line  Pai'k  and  Smithville,  and  also  upon 
the  shore  of  Lake  Superior  between  Split  Rock  Bay  and  Beaver  Bay. 

A  number  of  varieties  of  the  gabbro  were  seen  upon  Little  Saganaga 
Lake.     The  gabbro  varies  from  the  very  coarse-grained  varieties  to  forms 

"The  basic  massive  rocks  of  Lake  Superior,  by  W.  S.  Bayley:  Geol.  and  Nat.  Hist.  Survey  of 
Minnesota,  Final  Kept.,  Vol.  I,  1884,  pp.  688-716;  Vol.  II,  1888,  pp.  819-825;  Vol.  Ill,  1895,  pp.  1-20. 

6 Grant,  U.  S.  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV,  1899,  p.  447, 
PI.  MM,  figs.  5  and  6,  and  p.  489,  fig.  89. 

«Geol.  and  Nat.  Hist.  Survey  of  MinnesoUi,  Seventeenth  Ann.  Rept.,  1888,  p.  164. 


THE  KEWEENAWAN.  403 

which  have  rather  a  fine  grain,  these  two  grading  into  each  other.  In 
places  the  gabbro  becomes  so  feldspathic  that  it  can  be  correctly  spoken  of 
as  anorthosite.  These  anorthosite  masses  usualh-  weather  white,  and  being 
more  resistant  than  the  more  basic  gabbro  stand  out  as  bare,  white, 
conspicuous  knobs.  In  examining  these  anorthosite  masses,  which  are 
beautifully  exposed  in  numerous  places  on  the  islands  and  west  and 
southwest  shores  of  this  lake,  one  finds  scattered  through  them  irregular  and 
roundish  areas  of  what  appears  to  be  normal  gabbro.  This  grades  du'ectly 
into  the  anorthosite.  Furthermore  there  are  also  seen  finer-grained  facies 
of  the  gabbro  in  small,  rounded  areas  occurring  in  the  midst  of  the 
anorthosite  and  grading  into  the  surrounding  anorthosite.  It  thus  appears 
that  the  anorthosite  grades  both  into  the  normal  gabbro  of  coarse  grain 
and  also  into  the  normal  gabbro  of  fine  grain,  thus  showing  both  a 
mineralogic  and  textural  gradation.  The  more  basic  areas  which  are 
scattered  through  the  anorthosite  range  in  size  from  IJ  inches  in  diameter 
to  4  or  5  inches  in  diameter.  Between  these  basic  masses  lies  the 
anorthosite,  which  makes  up  the  greater  portion  of  the  rock,  covering 
much  larger  areas  than  are  occupied  by  the  basic  parts.  The  basic 
portions  weather  more  readily  than  the  anorthosite,  producing  a  pitted 
surface  upon  the  exposures.  When  disintegration  proceeds  much  farther, 
the  anorthosite  is  apt  to  break  down  into  rounded  bowlder-like  masses. 

In  many  places,  and  especially  near  the  northern  contact,  the  gabbro 
is  found  to  be  very  friable,  and  this  character  seems  to  be  due  to  a 
considerable  extent  to  some  character  of  the  rock  dependent  upon  its 
contact  with  the  adjacent  formations,  for  specimens  taken  farther  within 
the  mass  were  uniformly  fresher  and  harder. 

The  exposures  normally  show  a  rock  of  dark  color,  either  dark-reddish 
brown,  or  black,  varying,  as  is  stated  above,  to  the  anorthosite,  wliich  is  of 
rather  rare  occurrence,  and  has  a  gray  to  white  color.  The  other  extreme 
in  the  variation  from  the  anorthosite  is  represented  by  masses  consisting 
essentially  of  titaniferous  magnetite,  such  as  is  well  developed  at  Mayhew 
Lake"  and  especially  at  Iron  Lake.'' 

The  chief  components,  plagioclase,  feldspar,  pyroxene,  olivine,  titan- 
iferous magnetite,  are  clearly  recognized  in  hand  specimens.     With  these 

«W.  M.  Chauvenet,  TT.  S.  Geol.  Survey,  manuscript  notes. 

6Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  p.  489. 


404  THE  VERMILION  IRON-BEARIxNG  DISTRICT. 

are  found  occasionally  as   accessory  minerals,  liypersthene,   biotite,    and 
hornblende. 

3Iicroscopic  characters. — The  g-abbros  were  refen-ed  to  above  as  being 
generally  distiucth*  granular  rocks.  In  addition  to  the  granular  texture 
one  can  recognize  under  the  microscope  the  poikilitic  and  ophitic  textures. 
The  poikilitic  is  the  most  common,  and  in  rocks  possessing  this  texture  we 
find  plates  of  hornblende,  augite  less  commonly,  and  biotite  very  rarely, 
the  poikilitic  minerals  including  individuals  of  other  minerals.  The  ophitic 
texture  occurs  in  the  coarse  gabbros,  but  is  more  common  in  the  finer- 
grained  forms.  The  presence  of  this  ophitic  texture  in  these  rocks  is 
interesting  as  showing  the  gradations  in  textures  from  the  very  coarse 
gabbros  to  the  finer-grained  dolerites.  The  following  are  the  original 
constitutents  which  are  still  present  in  the  gabbro:  Augite,  liypersthene, 
olivine,  a  little  brownish-green  hornblende,  biotite,  apatite,  and  magnetite. 
The  plagioclase  has  been  determined  by  Bay  ley"  to  be  near  basic  labradorite 
in  character.  These  minerals  do  not  possess  any  very  exceptional  characters. 
Moreover,  they  have  been  described  in  great  detail  by  Bayley''  in  his 
articles  on  the  Duluth  gabbro  mass  of  northeastern  Minnesota.  The  only 
secondary  mineral  that  has  been  observed  is  the  serpentine.  The  rocks  are 
uormall)^  very  fresh  indeed.  They  vary  considerably  in  mineralogic 
composition.  In  some  cases  the  feldspar  is  practically  the  only  mineral 
present,  associated  with  only  occasional  grains  of  magnetite,  and  rounded 
individuals  of  augite,  forming  anorthosite.  With  the  feldspar  occurs 
occasionally  a  large  amount  of  olivine  and  some  titaniferous  magnetite. 
From  the  anorthosite  phase  the  rock  grades  through  facies  containing  the 
ferromagnesian  minerals  in  increasing  quantity  to  the  nearly  pure  titan- 
iferous magnetite  ores  as  the  extreme  variation.  Occasionally  the  rock 
consists  of  nearly  pure  augite  with  very  little  feldspar.  Sometimes  the 
biotite  is  present  in  considerable  quantity,  producing  the  biotite-gabbro. 
These  are  the  varieties  which  we  have  observed.  Many  details  concerning 
these  gabbros  are  given  by  Bayley"  in  the  papers  alread}-  referred  to.  The 
chemical  composition  of  the  gabbro  is  also  here  given.     These  analyses 


"The  basic  massive  rocks  of  the  Lake  Superior  region,  bj'  W.  S.  Bavlev:  Jour.  Geol.,  Vol.  I, 
1893,  p.  700. 

''The  basic  massive  rocks  of  the  Lake  Superior  region,  by  W.  S.  Bayley:  Jour,  (xeol..  Vol.  I, 
1893,  pp.  433-716;  Vol.  II,  1894,  pp.  814-825;  Vol.  Ill,  189.5,  pp.  i-20. 

t'Loc.  cit.,  p.  712. 


THE  KEWEENAW  AN. 


405 


were  made  for  Dr.  Bayley  in  the  Survey  laboratory,  and  were  reported  by 
him  in  the  papers  above  referred  to. 

No.    8589   contains  a  large  proportion  of  diallage  and  olivine,  while 
No.  8786  is  more  nearly  of  the  average  composition  of  the  entire  mass. 

Analyses  of  Duluth  gabbro. 


Constituent. 


8589. 


8786. 


SiO^  . 
TiOj. 

PA- 

AI2O3 

CrjO, 

FeO  . 

Fe,03 

NiO  . 

MnO 

CaO  . 

MgO. 

K,0. 


Na^O 

H2O  at  105°  -  - . . 
H„0  above  105° 


Total ;     100.03 


45.66 

46.45 

.92 

1.19 

.05 

.02 

16.44 

21.30 

Tr. 

13.90 

9.57 

.66 

.81 

.16 

.04 

Tr. 

Tr. 

7.23 

9.83 

11.57 

7.90 

.41 

.34 

2.13 

2.14 

.07 

.14 

.83 

1.02 

100. 03 

100.  75 

PETROGRAPHIC  CHARACTERS  OF  THE  LOGAN   SILLS. 

Macroscopic  characters. — The  rocks  forming  the  Logan  sills  are  nor- 
mally black,  medium-  to  coarse-grained  rocks,  although  varying  to  fine- 
grained aphanitic  facies  upon  the  edges  of  the  sills.  Occasionally  the  rock  is 
a  porphyry,  with  the  feldspars  as  the  phenocrysts.  Some  of  the  pheno- 
crysts  reach  4  inches  in  length,  but  they  are  normally  smaller,  ranging 
from  1  inch  to  2  inches.  Very  frequently  we  find  these  feldspars  collected 
into  large  masses  which  are  made  up  almost  entirely  of  these  minerals. 
Such  areas  usually  possess  as  the  result  of  weathering'  a  light  gray  or 
almost  white  color.  The  relatively  slightly  altered  masses  resemble  very 
closely  in  appearance  the  auorthosite  of  the  Duluth  gabbro  mass.  As  will 
be  seen  from  the  description  given  below,  the  sills  are  formed  of  rocks  which 
have  the  composition  and  characters  possessed  by  the  modern  dolerites,  and 
they  are  here  called  dolerites. 


4UG  THE  VERMILION  IRON-BEARING  DISTRICT. 

Microscopic  characters. — In  exceptional  cases  in  the  very  coarse-grained 
rocks  the  texture  is  almost  granular.  More  commonly  we  find  an  ophitic 
texture  imperfectly  developed,  with  the  feldspar  occurring  in  very  irregu- 
larl}-  bounded  but  in  general  lath-shaped  forms,  and  the  augite  in  more 
rounded  gi-ain-like  forms  than  is  common.  The  normal  ophitic  textui-e 
is  very  commonly  developed  in  these  rocks,  and  this  grades  over  into 
the  intersertal  texture  which  is  especially  well  developed  in  the  fine- 
grained border  facies.  The  fact  should  be  emphasized  that  while  the 
ophitic  texture  is  the  one  which  is  most  commonly  developed  in  these  rocks, 
there  is  occasionally  observed  an  imperfectly  ophitic  texture  grading  into 
a  more  or  less  gi'anular  texture,  and  this  will  be  referred  to  later  as 
evidence  in  favor  of  the  close  relationship  of  the  rocks  of  these  sills  with 
the  gabbro. 

Constituents. — The  rocks  are  very  fresh,  and  the  constituents  of  them 
are,  in  order  of  prominence,  augite,  feldspar,  titaniferous  magnetite,  then 
brownish-green  hornblende,  and  some  brown  biotite.  The  biotite  is  occa- 
sionally present  in  sufficient  quantity  to  warrant  our  speaking  of  the  rocks 
as  mica-dolerites.  The  only  secondaiy  mineral,  observed  is  a  light-green 
amphibole  derived  from  the  uralitization  of  the  augite.  The  rocks  are 
normal  dolerites,  as  shown  both  by  their  mineralog'ic  composition  and 
textures. 

RELATIONS    OF   THE    GABBRO    TO   ADJACENT    FORMATIONS. 

Within  this  district  the  gabbro  lies  adjacent  to  the  following  formations, 
given  in  order  of  age  from  below  up: 

The  Ely  greenstone  (Archean). 

Lower  Hiu'onian  sediments:  Ogishke  conglomerate  and  Knife  Lake 
slates. 

Giants  Range,  Snowbank,  and  Cacaquabic  granites. 

Upper  Huronian  sediments:  Grunflint  and  Rove  formations. 

Relations  to  the  Ely  greenstone. — The  gabbro  cuts  across  the  greenstone 
anticline  which  occurs  to  the  east  of  Disappointment  Lake,  and  is  also 
in  close  proximity  to  but  not  in  absolute  contact  with  the  greenstone 
of  the  Twin  Peaks  anticline  on  the  southwest  side  of  Gobbemichigamma 
Lake.  Li  both  of  these  cases  the  greenstone  has  been  metamorphosed  by 
the  gabbro,  showing  conclusively  the  relative  epoch  of  formation  of  the 
two  rocks. 


THE  KEWEENAWAN.  407 

Relations  to  Lower  Huronian  sediments. — At  a  number  of  places  which 
may  be  seen  by  reference  to  the  maps  in  the  athas  (Pis.  XV,  XVI)  the 
gabbro  is  in  contact  with  the  Lower  Huronian  sediments,  and  in  all  cases 
where  the  contacts  have  been  studied  the  sediments  have  been  found  to 
have  been  extremely  altered  as  the  result  of  their  proximity  to  the  gabbro. 
Minerals  have  been  produced  in  these  sediments  which  are  in  some  cases 
closely  related  to,  in  others  identical  with,  the  minerals  occurring  in  the 
gabbro  itself.  The  quantity  of  these  minerals  increases  as  the  gabbro 
is  neared,  and  all  evidence  points  unquestiouably  to  the  intrusion  and 
metamorphism  of  the  sediments  by  the  younger  gabbro. 

Relations  to  Giants  Range  granite. — The  Giants  Range  granite  and  the 
gabbro  occur  together  in  the  vicinity  of  the  Kawishiwi  River,  and  their 
relations  are  disclosed  by  a  dike  of  gabbro,  which  is  found  cutting  this 
granite  just  to  the  south  of  the  falls  in  sec.  19,  T.  64  N.,  R.  9  W.,  showing 
that  the  gabbro  is  younger  than  the  granite. 

Relations  to  Snowbank  and  Cacaquahic  granites. — No  contacts  have  been 
found  between  the  Snowbank  granite  and  the  gabbro,  or  between  the  Caca- 
quabic  granite  and  the  gabbro.  Since  both  of  these  granites  are  older  than 
the  Upper  Huronian  sediments,  which  we  know  have  been  intruded  by  the 
gabbro,  the  conclusion  is  evident  that  they  must  be  older  than  the  gabbro. 

Relations  to  Upper  Huronian  sediments. — The  gabbro  is  in  contact,  at  a 
number  of  places,  with  the  Upper  Huronian  sediments,  both  the  Gunflint 
formation  and  the  Rove  slates.  In  all  cases,  metamorphism,  which  the 
gabbro  has  produced  upon  these  sediments  as  the  result  of  its  contact,  offers 
conclusive  evidence  that  it  is  younger  than  they  are. 

Relations  to  the  Keweenaivan. — Irving  has  placed  this  gabbro  in  the 
Keweenawan  chiefly  as  the  result  of  his  studies  of  it  in  Wisconsin,"  and 
his  later  work  sustained  him  in  his  views,  as  is  shown  by  the  fact  that  after 
his  studies  in  Minnesota*  he  still  retained  it  in  its  same  stratigraphic  position. 
This  is  not  the  place  for  a  detailed  historical  review  of  the  various 
opinions  which  have  been  held  as  to  the  stratigraphic  position  of  the 
gabbro.  Reference  to  the  annual  reports  of  the  Minnesota  survey  will 
show  that  the  opinions  entertained  by  the  members  of  that  survey  as  to  its 


o  Geology  of  Wisconsin,  Vol.  IV,  p.  171. 

^The  copper-bearing  rocks  of  Lake  Superior  by  E.  D.  Irving:  Third  Ann.  Rept.  U.  S.  Gaol.  Sur- 
vey, 1883,  pp.  93-180.     Mon.  U.  S.  Geol.  Survey  Vol.  V. 


408  THE  VEKMILION  IRON-BEAKING  DISTRICT. 

relations  to  other  rocks  have  varied  greatly,  although  in  the  Final  Report 
(Vol.  IV)  it  is  regarded  as  of  Keweenawan  age,  in  general  agreement 
with  the  results  obtained  by  others  who  have  worked  upon  this  problem. 
Winchell"  makes  it  the  igneous  base  of  the  Keweenawan. 

As  the  result  of  work  done  during  the  field  season  of  1900  fairly 
conclusive  proof  has  been  obtained  of  the  fact  that  the  gabbro  is  in  reality 
intrusive  in  the  volcanic  Keweenawan,  and  consequently  younger  at  least 
than  a  portion  of  the  Keweenawan.  This  intrusive  relation  was  observed 
just  west  of  the  west  end  of  Brul^  Lake.  Brule'  Lake  lies  within  a  syncline 
of  Keweenawan  lavas,  bounded  on  the  south  and  west  by  the  Duluth  gabbro 
and  on  the  north  by  the  "red  rock"  of  the  Minnesota  survey.  From  this  great 
gabbro  mass  at  the  west  end  of  the  lake  an  eastward-projecting  tongue  of 
gabbro  was  traced  into  the  volcanics.  This  tongue  near  the  gabbro  pos- 
sessed the  normal  characters  of  the  main  gabbro  mass,  but  to  the  east 
it  narrowed  rapidly  and  its  lithologic  characters  changed  until  where  nar- 
rowest, just  before  it  disappeared  in  a  topographic  depression,  it  had  graded 
into  a  porphyritic  rock  of  comparatively  fine  grain,  with  a  selvage  which 
was  very  nearly  basaltic.  The  lavas  were  upon  both  sides  of  this  tongue. 
The  actual  contact  between  the  gabbro  and  the  lava  was  not  found,  but 
they  were  separated  at  one  place  by  an  interval  of  only  about  1  foot, 
and  this  was  the  place  where  the  tongue  showed  its  finest  grain.  The 
connection  of  this  tongue  with  the  gabbro  and  the  variation  in  grain  in 
the  tongue  seem  to  be  very  good  evidence  of  the  intrusive  character  of  the 
gabbro. 

RELATIONS  OF  THE  LOGAN   SILLS  TO  ADJACENT   FORMATIONS. 

Relations  to  the  Upper  Huronian  (^Animikie). — Up  to  the  time  of  the 
publication  in  1893  of  Lawson's*  paper  on  the  laccolitic  sills  of  the 
northwest  coast  of  Lake  Sujjerior,  all  of  the  earlier  writers  on  the  Lake 
Superior  region,  with  the  exception  of  Irving"  and  Ingall''  had  held  the 
sheets  of  basic  rock  which  they  observed  intercalated  between  the  Huronian 
slates  to  be  of  volcanic  origin — that  is,  surface  flows  interbedded  with  the 

«Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  p.  298. 

''The  laccolitic  sills  of  the  northwest  coast  of  Lake  Superior,  by  A.  C.  Lawson:  Geol.  and  Nat. 
Hist.  Survey  of  Minnesota,  Bull.  No.  8,  pt.  2, 1893,  pp.  24-48  See  this  for  references  to  writers  giving 
details  of  sills  which  can  not  be  given  in  present  paper. 

«Mon.  U.  S.  Geol.  Survey  Vol.  V,  1883,  p.  379. 

''Geol.  and  Nat.  Hift.  Survey  Canada,  Ann.  Kept.,  1888,  H,  p.  25. 


THE  KEWEENAWAN.  409 

sediments.     lug-alls  held  that  some  of  these  masses  were  of  intrusive  origin. 
Irving  considered  them  all  as  intrusive. 

Chauveuet,  in  his  manuscript  notes,  refers  to  a  dike  which  he  places 
with  the  Logan  sills,  cutting-  the  Animikie  slates  on  Pig*eon  River.  This 
rock  is  clearly  an  intrusive  dike  in  the  slates.  The  edges  of  the  slates  next 
to  the  dike  are  much  shattered  and  broken,  as  the  result  of  this  intrusion. 

In  the  Minnesota"  reports  these  sills  are  referred  to  as  intrusions  in  the 
Animikie  slates,  but  are  included  in  the  description  of  the  Animikie  which 
corresponds  to  our  Upper  Huronian  Series. 

Lawson*  asserts  that  these  sills  are  all  intrusive,  and  fortifies  his  state- 
ment by  proof  which  seems  to  be  unquestionable,  as  the  following-  quota- 
tions of  the  summary  of  his  argument  from  his  paper  will  show: 

I.  The  trap  sheets  associated  ^vith  the  Animikie  strata  are  not  volcanic  flows, 
because  of  the  combination  of  the  following-  facts: 

1.  They  are  simple  geological  units,  not  a  series  of  overlapping-  sheets. 

2.  They  are  flat  with  uniform  thickness  over  areas  more  than  100  square  miles 
in  extent,  and,  where  inclined,  the  dip  is  due  essentially  to  faulting  and  tilting. 

3.  There  are  no  pja-oclastic  rocks  associated  with  them. 
•i.  They  are  never  glassy. 

5.  They  are  never  amygdaloidal. 

6.  They  exhibit  no  flow  structure. 

7.  They  have  no  ropy  or  wrinkled  surface. 

8.  They  have  no  lava-breccia  associated  with  them. 

9.  They  fcame  in  contact  with  the  slates  after  the  latter  were  hard  and  brittle, 
and  had  acquired  their  cleavage;  yet  they  never  repose  upon  a  surface  which  has 
been  exposed  to  subaerial  weathering. 

II.  They  are  intrusive  sills,  because  of  the  combination  of  the  following  facts: 

1.  They  are  strictty  analogous  to  the  great  dikes  of  the  region:  (a)  In  their 
general  relations  to  the  adjacent  rocks,  and  in  their  field  aspect;  (i)  in  that  both  the 
upper  and  lower  sides  of  the  sheets  have  the  facies  of  a  dense  aphanitic  rock,  which 
grades  toward  the  middle  into  a  coarsely  crystalline  rock. 

2.  They  have  a  practicallj^  uniform  thickness  over  large  areas. 

3.  The  columnar  structure  extends  from  lower  surface  to  upper  surface,  as  it 
does  from  wall  to  wall  in  the  dikes. 

rt.  They  intersected  the  strata  above  and  below  them  after  the  latter  had  been 
hard  and  brittle. 

6.  They  may  be  obsei-ved  in  direct  continuity  with  dikes. 
6.  They  pass  from  one  horizon  to  another. 

"Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  pp.  476;  487-488. 
*Lawson,  loc.  cit.,  p.  29. 


410  THE  VERMILION  IRON-BEARING  DISTRICT. 

8[7].  The  bottom  of  the  sedimentary  strata  above  them,  wherever  it  is  observable, 
is  a  freshly  ruptured  surface. 

9[8].  Apophyses  of  the  trap  pass  from  the  main  sheet  into  the  cracks  of  the  slate 
above  and  below. 

10[9].  The  trap  sheets,  particularh'  at  the  upper  contact,  hold  included  fragments 
of  the  overlying  slates. 

11[10].  They  locally  alter  the  slates  above  and  below  them." 
The  writer  does  not  believe  that  the  faulting-  sug-gested  in  I,  2,  above, 
explains  the  structure  of  the  rocks,  as  no  evidence  in  favor  of  this  faulting- 
has  been  observed.  Facts  have  been  observed  in  the  Lake  Superior  region 
which  are  corroborative  of  Lawson's  other  statements  in  all  their  essentials, 
and  confirm  his  conclusions. 

Belations  of  sills  to  Keweenawan. — Lawson''  goes  still  further,  and  draws 
the  important  conclusion  from  evidence  observed  during-  his  field  work  that 
these  sills  are  not  only  later  than  the  Animikie  sediments,  but  are  intrusive 
in  the  Keweenawan,  and  states  it  as  his  opinion  that  they  are  identical  in 
age  with  many  of  the  heavy  sheets  of  dark  diabase  or  gabbro  which  prevail 
on  the  Minnesota  coast,  particularly  in  the  eastern  portion.  He  therefore 
places  the  sills  as  post-Keweenawan,  and  possibly  of  Silurian  age.  The 
writer  dissents  altogether  from  the  idea  that  these  sills  and  dikes  can  be 
post-Keweenawan,  since  nowhere  in  the  Lake  Supei'ior  region  have  the 
Cambrian  rocks  been  found  to  be  cut  by  dolerite  sills  and  dikes,  or  indeed 
by  any  intrusives.  While  the  sills  may  be  younger  than  part  of  the 
Keweenawan,  there  is  no  reason  for  supposing  them  younger  than  all  of 
the  Keweenawan,  but  they  may  belong  to  the  same  period  of  igneous 
activity  and  be  merely  one  of  the  later  expressions  of  this  activity,  when 
the  molten  magma  was  too  deeply  buried  to  reach  the  surface  freely  as 
flows,  and  was  intruded  between  the  sediments  of  the  Animikie  and  the 
lava  sheets  of  the  Keweenawan  wherever  conditions  were  favorable.  Thus 
considered  they  would  be  of  Keweenawan  age  instead  of  post-Keweenawan, 
as  has  been  supposed  by  Lawson. 

Belations  of  the  gabbro  to  the  Logan  sills. — The  question  of  the  relation- 
ship of  the  gabbro  to  the  Logan  sills  has  been  discussed  by  Grant,  who 
states  that  he  "is  inclined  to  separate  the  sills  from  the  gabbro,  but  admits 
that  this  separation  is  not  proven."     The  various  facts  which  he  considers 


"Lawson,  loi:.  cit.,  pp.  44-45. 

i  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Bull.  No.  8,  1893,  p.  47. 


THE  KEWEENAW  AN.  41 1 

as  evidence"  against  the  relationship  of  the  two  kinds  of  rocks  he  summarizes 
as  follows: 

1.  The  sills  are  considerably  altered,  i.  e.,  the  pyroxene  has  usually  largely 
been  replaced  by  secondary  nainerals,  while  the  gabbro  is  usually  fresh  and  the 
olivine  as  well  as  the  pyroxene  is  usually  unaltered. 

2.  The  sills  are  essentially  nonolivinitic;  at  least  traces  of  olivine,  even  when 
altered,  are  not  common.     The  gabbro  is  normallv  olivinitic. 

3.  The  sills  are  quite  rich  in  ferromagnesian  minerals,  giving  a  dark-gra}'  or 
black  color  to  the  rock.  The  gabbro  is  usually  rich  in  feldspar  and  rather  poor  in 
ferromagnesian  minei'als,  and  the  rock  is  light  gray  in  color.  When  a  basic  mineral 
predominates,  it  is  mostly  iron  ore,  which  is  not  the  case  with  the  sills. 

i.  The  sills  are  in  structure  ophitic;  the  gabbro  is  granitic.  This  holds  true 
also  of  the  coarsest-grained  sills  and  of  the  finest-grained  gabbro.  In  this  connection, 
it  might  be  well  to  mention  some  sills  in  the  Animikie  at  Akeley  Lake  in  the  Akeley 
Lake  plate;  these  are  apparently  of  gabbro.  They  are  fine  grained  at  their  edges, 
but  even  here  the  structure  is  more  nearly  that  of  the  gabbro,  and  not  that  of  the 
ordinary  sills. 

5.  The  sills  are  very  fine  grained,  almost  glassy  at  the  lower  and  upper  sides, 
even  in  the  thickest  sills.  The  gabbro  is  not  very  fine  grained  at  the  contact  with 
the  Animikie  rocks,  and  even  on  the  edges  of  the  apparent  gabbro  sills  mentioned 
above,  the  fineness  of  the  grain  nowhere  approaches  that  of  the  edges  of  the  ordinary 
sills. 

6.  The  sills,  even  the  largest  ones,  have  macroscopically  altered  the  Animikie 
rocks  for  only  a  very  few  feet,  or  even  inches,  from  the  contacts,  while  the 
metamorphism  of  the  Animikie  at  the  gabbro  contact  is  profound,  extending  for  a 
distance  of  several  rods. 

7.  The  gabbro  and  sills  Tiave  not  been  traced  together,  neither  have  they  been 
found  in  contact.  In  the  map  the  sills  have  not  been  shown  in  contact  with  the 
gabbro;  this  is  on  account  of  lack  of  exposures.  They  may,  of  course,  come  into 
actual  contact  with  the  gabbro. 

8.  Where  the  sills  and  the  gabbro  come  nearest  together  the  two  rocks  are  easily 
distinguished,  even  in  the  field.  The  few  specimens  about  which  there  is  question 
have  been,  as  far  as  examined  microscopically,  easily  referred  to  one  or  the  other. 

In  closing  his  summary  Grant  says  that  he  is  incliaed  to  the  idea  that 
these  sills  are  of  earlier  date  than  the  gabbro. 

Several  37'ears  before  the  publication  of  the  above  statement  Lawson 
expressed  his  opinion  *  that  the  sills  are  identical  with  many  of  the  heavy 
sheets  of  dark  diabase  and  gabbro  which  prevail  on  the  eastern  part  of  the 
Minnesota  coast,  and  which  are  associated  with  the  Keweenawan.     The 

aGeol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  p.  488. 
i  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Bull.  No.  8,  1893,  p.  47. 


412  THE  VERMILION  IRON-BEARING  DISTRICT. 

writer  is  prepared  to  go  much  farther  than  Lawson,  and  to  express  the 
opinion  that  these  sills  are  the  equivalent  in  age  of  the  Duluth  gabbro  of 
northeastern  Minnesota,  and  even  that  they  were  derived  from  the  same 
magma  from  which  it  was  derived.  In  the  following  paragraphs  Grant's 
statement,  as  quoted  above,  will  be  discussed  and  evidence  will  be  presented 
to  support  the  above-mentioned  view  of  the  intimate  relationship  of  the  sills 
and  gabbro.  This  evidence  was  collected  partly  during  work  on  the  Ver- 
milion district  and  during  several  reconnaissance  trips  into  the  Keweena- 
wan  gabbro  from  the  Vermilion  district,  in  the  course  of  a  portion  of  a 
season's  field  work  on  the  Keweenawan  of  Minnesota  and  Canada,  made  in 
1900. 

No.  1.  The  sills  are  considerably  altered,  i.  e.,  the  pyroxene  has  usually  larg-ely 
been  replaced  by  secondary  minerals,  while  the  gabbro  is  usualh'  fresh  and  the 
olivine  as  well  as  the  pyroxene  is  usually  unaltered. 

The  facts  are  essentially  coiTect,  and  no  explanation  can  be  ofi'ered  for 
this  difference,  unless  it  is  that  it  results  from  the  sills  being  of  smaller 
mass  and  intercalated  in  the  slates  and  having  been  exposed  in  consequence 
to  a  more  energetic  action  of  water  than  has  the  gabbro.  The  gabbro 
disintegrates  very  easily,  and  even  when  the  state  of  aggregation  is  such 
that  the  rock  can  be  easily  crushed  in  the  hand  the  constitutents  are  rela- 
tively fresh.  This  gabbro,  as  a  result  of  this  readiness  to  disintegrate,  has 
had  its  outer  disintegrated  portion  removed  by  glacial  action  and  water 
erosion.  In  general,  erosion  has  kept  pace  with  the  disintegration  of  the 
gabbro.  Hence  the  rock  which  we  now  observe  is  very  fresh.  The  sills, 
on  the  other  hand,  are  very  much  more  resistant.  All  those  examined 
were  fairly  fresh  and  exceedingly  hard  and  tough.  If  one  could  get  a 
specimen  from  the  rocks  of  the  sills  deep  down  in  the  mass,  doubtless  the 
rock  would  be  found  to  be  about  as  fresh  as  the  ffabbro. 


to"- 


2.  The  sills  are  essentially  nonolivinitic;  at  least  traces  of  olivine,  even  when 
altered,  are  not  common.     The  gabbro  is  normally  olivinitic. 

This  is  also  true.  But  one  must  not  neglect  the  fact  that  locally  the 
gabbro  is  also  practically  free  from  olivine.  The  local  absence  of  olivine 
in  the  gabbro  is  due  to  conditions  of  crystallization  of  the  gabbro  magma 
and  possibly  slight  chemical  differences  also,  and  just  in  the  same  way  can 
one  explain  the  absence  of  olivine  from  the  sills.     No  analyses  have  been 


THE  KEWEENAWAN.  413 

made  of  the  various  facies  of  the  gabbro  to  determine  the  chemical  differ- 
ences which  may  exist  between  them,  nor  have  analyses  been  obtained  of 
the  rock  of  the  sills  to  prove  its  identical  chemical  composition  with  the 
gabbro  as  a  whole  or  with  any  of  its  special  facies. 

3.  The  sills  are  quite  rich  in  ferromagnesian  minerals,  giving  a  dark -gray  or 
black  color  to  the  rock.  The  gabbro  is  usually  rich  in  felds2:)ar  and  rather  poor  iu 
ferromagnesian  minerals,  and  the  rock  is  light  gray  in  color.  When  a  basic  mineral 
predominates,  it  is  mostly  iron  ore.  which  is  not  the  case  with  the  sills. 

The  writer  does  not  concur  in  this  statement.  The  fact  that  the  sills 
are  generally  darker  than  the  gabbro  is  due  largely  to  tlie  finer  grain  of  the 
sill  rocks.  When,  however,  the  sills  are  very  coarse  grained  one  finds 
patches  that  are  made  up  almost  exclusively  of  feldspar  in  large  individuals 
and  such  areas  iu  the  sills  are  a  very  light  gray  and  become,  when  the  feldspar 
is  kaolinized,  nearly  snow  white.  On  the  other  hand,  some  of  the  sections 
from  the  sills  which  have  been  examined  are  made  up  largely  of  magnetite, 
with  pyroxene  second  in  abundance,  and  last,  the  feldspar.  This  proportion 
of  minerals  is  not  the  rule  in  the  sills,  but  neither  is  it  the  rule  in  the  gabbro, 
as  witness  the  light-gray  anorthosite  with  but  little  of  the  ferromagnesian 
compounds. 

4.  The  sills  are  in  structure  ophitic;  the  gabbro  is  granitic.  This  holds  true  also 
of  the  coarsest-grained  sills  and  of  the  linest-grained  gabbro.  In  this  connection  it 
might  be  well  to  mention  some  sills  in  the  Animikie  at  Akeley  Lake,  in  the  Akeley 
Lake  plate;  these  are  apparentl}'  of  gabbro.  They  are  fine  grained  at  their  edges, 
but  even  here  the  structure  is  more  nearly  that  of  the  gabbro  and  not  that  of  the 
ordinary  sills. 

The  writer  must  disagree  with  the  above  statement  of  the  texture  of  the 
rocks,  as  his  own  studies  have  shown  that  while  the  gabbro  is  predomi- 
nantly granitic,  nevertheless  the  ophitic  texture  is  very  frequent  in  the 
finer-grained  facies.  On  the  other  hand,  the  sills  show,  in  places  where  the 
rock  is  as  coarse  as  is  the  finer-grained  gabbro,  a  distinctly  ophitic  texture, 
grading  into  an  imperfectly  granular  one,  and  from  this  down  into  very 
fine-grained  intersertal  textured  basalts.  A  porphyritic  texture  which  was 
not  observed  in  the  g-abbros  is  common  in  the  sills.  These  facts  indicate  a 
textural  gradation  between  the  gabbro  and  sills,  the  differences  in  general 
textural  characters  being  due  to  the  difference  in  the  conditions  between 
the  slow  crystallization  of  an  enormous  mass  of  magma,  as  in  the  case  of 


414  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  gabbro,  and  of  a  relatively  small  mass  and  quick  cooling  and  crystal- 
lization, as  in  the  case  of  the  sills.  This  difference  in  the  rapidity  of  cool- 
ing is  shown  also  by  the  porphyritic  structure  of  the  rocks  of  the  sills  and 
the  fine-grained  rock  occurring  upon  the  edges  of  the  sills.  The  relative 
rapidity  of  the  cooling  is  further  indicated  by  the  feldspar  pheuocrysts 
which  occur  in  the  sills.  These  very  commonly  reach  a  length  of  2  inches, 
with  a  maximum  length  of  4  inches.  Had  the  magma  cooled  under  the 
conditions  under  which  the  phenocrysts  were  formed,  obviously  the  rest  of 
the  constituents  would  have  also  attained  much  larger  size  than  they  did, 
and  the  resulting  rock  would  have  been  coarse  grained  and  doubtless  as 
granular  as  any  of  the  gabbro. 

But  let  us  not  neglect  the  evidence  presented  in  the  above-quoted  state- 
ment itself — that  offered  in  the  statement  that  there  are  sills  at  Akeley 
Lake  which  are  apparently  of  gabbro  and  not,  like  the  other  sills,  different 
from  the  gabbro.  Let  us  attempt  to  conceive  of  the  conditions  under  which 
these  sills  were  formed.  They  were  injected  into  the  sediments  only  a  short 
distance  away  from  the  edge  of  the  main  gabbro  mass,  and  have  been  traced 
parallel  with  this  edge  for  a  number  of  miles.  Every  observer  of  the  rocks 
in  which  these  sills  lie  intercalated  states  that  the  rocks  are  those  which  show 
the  most  extreme  effects  of  the  gabbro  contact  action.  They  are  metamor- 
phosed so  that  it  is  nearly  impossible  to  determine  their  original  character. 
Evidently  they  were  exposed  to  the  high  temperature  of  the  adjacent  gab- 
bro for  a  long  time,  and  the  magma  of  the  sills  must  have  profited  by  this 
high  temperature  of  the  parent  mass  of  magma  and  the  heated  sediments 
when  it  was  intruded,  and  cooled  more  slowly  than  it  otherwise  would  have 
done,  and  much  more  slowly  than  the  sills  farther  from  the  parent  mass. 
Hence  the  sill  rock  approaches  in  its  texture  much  more  closely  that  of 
the  parent  mass  of  gabbro.  No  reason  can  be  seen  why  these  sills  should 
be  connected  with  the  gabbro  as  gabbro  sills  and  separated  from  the  other 
sills  in  their  vicinity,  and  even  farther  away,  which  occur  under  practi- 
cally identical  conditions,  and  show  but  relatively  unimportant  petrographic 
differences  from  them. 

5.  The  sills  are  very  fine  grained,  almost  glass_y  at  the  lower  and  upper  sides, 
even  in  the  thickest  sills.  The  gabbro  is  not  verj'  fine  grained  at  the  contact  with 
the  Animikie  rocks,  and  even  on  the  edges  of  the  apparent  gabbro  sills  mentioned 
above,  the  fineness  of  grain  nowhere  approaches  that  of  the  edges  of  the  ordinary  sills. 


THE  KEWEENAW  AN.  415 

This  difference  in  grain  of  the  rock  on  the  edge  of  the  sill  and  of  that 
on  the  edge  of  the  main  mass  of  the  gabbro  is  the  difference  which  normally 
occurs  where  there  is  such  a  great  discrepancy  in  the  size  of  the  masses  as 
there  is  in  the  case  under  consideration.  Witness,  for  example,  the  character 
of  the  rock  at  the  edge  of  any  very  large  granite  massive  and  that  upon 
the  edge  of  the  dikes  radiating  from  it.  Undoubtedly,  however,  the  gabbro 
is  materially  finer  grained  on  the  periphery  of  the  mass  than  it  is  in  the 
interior,  and  there  is  an  imperceptible  gradation  between  the  fine  and 
the  coarse  facies.  Moreover,  the  border  facies  of  the  gabbro  is  about  of 
the  same  degree  of  coarseness  or  possibly  even  less  coarse  than  the  rock 
occui'ring  in  the  interior  of  the  thickest  sills.  From  the  nature  of  the 
occurrence  one  should  not  expect  the  border  of  the  gabbro  to  be  as  fine  as 
the  rock  upon  the  edge  of  the  sills. 

6.  The  sills,  even  the  largest  ones,  have  macroscopically  altered  the  Animikie 
rocks  for  only  a  very  few  feet,  or  even  inches,  from  the  contacts,  while  the  meta- 
morphism  of  the  Animikie  at  the  gabbro  contact  is  profound,  extending  for  a  distance 
of  several  rods. 

The  relative  intensity  of  the  metamorphic  action  of  the  gabbro  and  sills 
depends  largely,  as  does  the  size  of  the  grain  of  the  rocks,  upon  the  masses 
of  the  magma,  since  this  influences  the  rate  of  cooling.  Of  necessity  a  small 
mass  of  rock  like  a  sill  would  have  less  effect  upon  the  sediments  than 
the  gabbro. 

One  is  better  prepared  to  appreciate  the  difference  in  the  effect  of  the 
sills  and  main  gabbro  mass  when  one  thinks  that  the  thickest  sills  are  only 
about  400  feet  thick,  and  that  these  are  utterly  insignificant  when  compared 
with  the  Dviluth  gabbro  mass  which  covers  about  2,400  square  miles  in 
Minnesota." 

7.  The  gabbro  and  sills  have  not  been  traced  together;  neither  have  thej^  been 
found  in  contact.  In  the  map  the  sills  have  not  been  shown  in  contact  with  the 
gabbro;  this  is  on  account  of  lack  of  exposures.  Thej'  may,  of  course,  come  into 
actual  contact  with  the  gabbro. 

It  is  true  that  no  actual  contacts  of  gabbro  and  sill  ha.ve  been 
observed,  although  they  have  been  seen  separated  only  by  a  short  distance. 
At  one  place  on  the  Duluth,  Port  Arthur  and  Western  Railroad,  between 

"The  geology  of  the  Keweenawan  area  in  northeastern  Minnesota,  III,  Pt.  II,  Geology  of  the 
Keweenawan  series,  by  A.  H.  Elftman:  Am.  Geologist,  Vol.  XXII,  1898,  p.  132. 


416  THE  VERMILION  IRON-BEARING  DISTRICT. 

the  first  and  second  trestles  east  of  Paulson's  mine,  there  is  a  small  mass  of 
crumpled,  much  metamorphosed  black  slate,  with  the  main  mass  of  the 
gabbro  to  the  south,  and  a  small  mass  of  gabbro  to  the  north.  This  slate 
is  considered  by  Grant  to  be  an  inclusion  in  the  gabbro.  The  writer  is 
inclined  to  think  this  slate  is  caught  in  the  fork  between  the  main  gabbro 
mass  and  a  sill  which  is  an  offshoot  from  it.  Lack  of  exposures  prevented 
the  tracing  of  the  connection  between  them.  It  must  be  said  in  this 
connection  that  in  our  work  on  the  Vermilion  iron  district  this  relation 
between  the  gabbro  and  the  sills  was  considei'ed  a  problem  of  importance 
secondary  to  that  of  mapping  the  iron-bearing  formations,  and  no  especial 
attempt  was  made  to  trace  the  relation  between  these  rocks  in  the  field.  It 
is  believed  that  this  connection  would  be  shown  to  exist  if  the  edge  of  the 
gabbro  and  the  adjoining  Auimikie  area  were  mapped  in  detail. 

In  this  connection  also  it  seems  that  the  fact  that  the  srabbro  and  sills 
have  not  thus  far  been  found  in  contact  cotild  be  used  as  evidence  more 
strongly  against  the  sills  being  older  than  the  gabbro,  as  Grant"  suggests, 
since  the  gabbro  is  found  in  contact  with  every  other  rock  in  the  district 
which  has  been  proved  to  be  older  than  it  is.  If  the  sills  are  older  than 
the  gabbro,  they  would  form  the  one  exception. 

8.  AVhere  the  sills  and  the  gabbro  come  nearest  together  the  two  rocks  are 
easily  distinguished,  even  in  the  field.  The  few  specimens  about  which  there  is 
question  have  been,  as  far  as  examined  microscopically,  easity  referred  to  one  or  the 
other. 

Exception  is  taken  to  this  statement,  and  the  reader  is  referred  to 
section  4  above,  where  certain  sills  are  mentioned  in  these  words: 

In  this  connection  it  might  be  well  to  mention  some  sills  in  the  Animikie  at 
Akeley  Lake  in  the  Akeley  Lake  plate;  these  are  appai'eutl}' of  gabbro.  They  are 
fine-grained  at  their  edges,  but  even  here  the  structure  is  more  nearly  that  of  the 
gabbro,  and  not  that  of  the  ordinary  sills. 

The  rock  of  the  sills  can  in  general  be  readily  distinguished  from  that 
of  the  main  gabbro  mass,  but  so  can  the  dike  rocks  of  rhyol-ite,  etc.,  from 
the  coarse  granite  of  the^massives  from  which  demonstrably  the  dikes  are 
offshoots.  This  difference  is  not  evidence  of  importance  against  the  rela- 
tionship of  the  sills  and  of  the  gabbro. 

«Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV.  1899,  p.  4SS. 


THE  KEWEENAW  AN.    .  417 

CONCLUSIONS  AS  TO  AGE  AND  RELATION  OF  THE  GABBRO  AND  SILLS. 

In  conclusion,  there  is  presented  the  following  summary  of  the  facts 
observed  which  seem  to  indicate  the  true  relationship  between  the  gabbro 
and  the  Logan  sills  and  their  correct  stratigraphic  position.  The  gabbro 
and  the  rock  of  the  sills  are  petrographically  the  same  and  textural  gTada- 
tions  have  been  observed  which  indicate  their  close  relationship.  The 
gabbro,  while  predominantly  coarse-grained  and  granular,  is  locally  fine- 
grained and  poikilitic,  and  in  one  place  was  found  as  a  dike  in  the  Kewee- 
nawan  and  there  graded  into  a  porphyritic  facies  and  even  into  a  fine- 
grained ophitic  dolerite.  The  rock  of  the  sills  is  in  places,  in  the  midst 
of  the  thick  sills,  a  good  granular  gabbro  in  texture,  and  ranges  from  this 
through  ophitic  and  poikilitic-textjired  dolerites  into  fine-grained  aphanitic 
intersertal-textured  basalts  upon  the  selvage.  Mineralogically  they  are  the 
same,  excepting  that  in  the  relatively  few  specimens  from  the  sills  which 
have  been  studied  no  olivine  nor  hypersthene  has  been  observed,  nor  do 
the  sills  show  such  great  miueralogic  variation  from  titaniferous  magnetite 
rocks  to  enormous  anorthosite  masses,  although  there  ai-e  small  anorthosite 
masses  in  the  sills.  Such  differences  in  variation  are,  however,  easily 
explicable  as  due  to  the  enormous  difference  existing  between  the  masses 
of  magma  forming  the  gabbro  and  that  forming  the  indi-\ddual  sills. 

It  is  admitted  by  all  that  both  the  gabbro  and  the  sills  are  younger 
than  the  Upper  Huronian,  since  they  both  have  been  observed  at  numbers 
of  places  to  cut  the  rocks  of  this  age.  The  point  of  difference  is  the  rela- 
tionship between  the  gabbro  and  sills  and  the  Keweenawan.  The  writer 
found  the  gabbro  cutting  a  portion  of  the  Keweenawan  rocks,  whose  exact 
stratigraphic  position  with  relation  to  the  remainder  of  the  Keweenawan  is, 
however,  not  known,  and  hence  considers  it  and  the  sills  as  younger  than 
some  of  the  Keweenawan. 

The  gabbro  is  believed  to  be  a  great  laccolitic  mass  which  in  general 
follows  approximately  the  contact  plane  between  the  Animikie  series 
(Upper  Huronian)  and  the  Keweenawan.  In  the  Vermilion  district  there 
are  local  departures  from  this  which  will  be  described  in  the  following- 
paragraph. 

Over  a  great  part  of  the  southern  edge  of  the  Vermilion  district  the 
gabbro  followed  essentially  along  the  plane  between  the  Upper  Huronian 
(Animikie)  and  the  lower  lying  sediments,  uplifting  thereby  the  Upper 
MON  XLV — 03 27 


418  THE  VERMILION  IRON-BEARING  DISTRICT. 

Huronian  sediments,  for  at  several  places  on  the  edge  of  the  Vermilion  dis- 
trict and  just  south  of  it  are  found  isolated  patches  of  the  lowest  part  of 
the  Guuflint  formation  (Animikie)  included  in  the  Keweenawan  gabbro. 
The  gabbro  in  this  part  of  the  district  may  originally  have  been  completely 
covered  by  the  Animikie  series,  but  in  this  part  of  the  district,  also,  the 
rocks  are  much  more  closely  folded  than  in  the  east  near  Gunflint  and  in 
the  west  in  the  Mesabi  district,  and  as  a  result  of  the  erosion  in  this  area, 
where  the  rocks  have  been  folded  and  fractm-ed,  all  of  the  Upper  Huronian 
(Animikie)  but  the  few  included  patches  has  been  removed. 

In  the  eastern  part  of  the  Vermilion  district  the  gabbro  began  to  rise 
and  cut  across  the  Upper  Huronian,  reaching  higher  and  higher  beds  to  the 
east,  and  then  spread  out  essentially  along  the  plane  between  the  Upper 
Huronian  (Animikie)  and  the  base  of  the  Keweenawan,  sending  sills  and 
dikes  into  the  Rove  slates  of  the  Upper  Huronian  and  also  into  the 
Keweenawan  rocks,  as  can  be  seen  on  Brule  Lake. 

The  writer  is  inclined  to  believe  that  the  gabbro  is  a  great  basic 
igneous  mass  which  represents  a  basic  differentiation  product  of  a  magma 
from  which  perhaps  the  major  portion  of  the  Keweenawan  volcanics  were 
derived,  and  which  basic  magma  has,  perhaps,  as  its  complementary  acid 
rocks,  the  great  mass  of  intrusive  "Red  Rock"  and  the  related  rhyolite 
flows  of  the  Keweenawan. 

METAMORPHIC   EFFECT   OF    GABBRO   AND    SILLS. 

The  effect  of  the  gabbro  upon  the  various  rocks  with  which  it  is  in 
contact  has  been  considered  under  the  description  of  those  various  rocks, 
for  example,  under  the  Ely  greenstone,  the  Rove  slates,  etc.,  but  a  brief 
summary  statement  will  be  made  here  concerning  the  character  of  this 
metamoi'phism.  The  most  noticeable  general  effect  of  the  gabbro  has 
been  to  produce  a  very  large  quantity  of  rich  brown  biotite  in  the  rocks  in 
contact  with  it. 

Archean  {Ely)  greenstones. — In  the  meta-dolerites  and  meta-basalts 
(greenstones)  the  general  effects  are  much  the  same  as  in  the  case  of  the 
sediments.  The  ophitic  texture  of  those  greenstones  which  have  been 
studied  is  still  retained  with  a  fair  degree  of  distinctness.  There  is,  how- 
ever, a  tendency  to  gradually  destroy  the  texture  and  produce  granular 
rocks  therefrom  by  the  production  of  large  quantities  of   biotite,   with 


THE  KEWEENAW  AN.  419 

liyperstheue,     augite,    brownish-green    hornblende,    and    magnetite,    and 
possibly  the  recrystallization  of  the  feldspar. 

Lower  Huronian. — In  the  Lower  Huronian  sediments  this  production 
of  biotite  has  been  accompanied  by  a  recrystaUization  of  the  minerals  of 
the  rock,  whereby  the  sedimentary  structures  are  mostly  destroyed  and 
mica-schists  and  gneisses  are  produced.  Immediately  along  the  edge  of 
the  contact  with  these  schists  in  the  metamorphosed  sediments  occur  large 
quantities  of  ferromagnesian  minerals,  such  as  augite,  hypersthene,  and 
brown  hornblende.  In  the  case  of  the  conglomerates  we  find,  since  there  are 
a  number  of  different  kinds  of  pebbles,  and  these  differ  from  the  matrix  in 
which  they  lie,  that  the  pebbles  when  affected  furnish  a  somewhat  different 
product  from  the  matrix,  and  this  product  usually  stands  weathering  better 
than  the  matrix,  so  that  the  pebbly  character  of  the  conglomerate  is  retained. 

Upper  Huronian  (Gunflint  formation). — Where  the  gabbro  has  been  in 
contact  with  or  very  near  the  rocks  of  the  iron-bearing  Gunflint  formation, 
it  has  affected  them  in  a  very  marked  way,  and  has  produced  magnetite 
ores  interlaminated  with  bands  of  qiaartz  and  a  rock  composed  of  olivine, 
hypersthene,  augite,  hornblende,  and  magnetite,  with  quartz  in  varying 
proportions.  The  rocks  thus  produced  are  different  from  any  original 
igneous  or  sedimentary  rocks  with  which  the  writer  is  acquainted. 

Bove  slates. — The  metamorphism  of  the  Rove  slates  by  the  gabbro 
has  been  such  as  to  produce,  from  rocks  consisting  of  angular  grains  of 
quartz,  feldspar,  and  plates  of  chlorite,  and  a  dark  interstitial  material, 
more  or  less  completely  crystallized  mica-schists,  or,  with  larger  percentage 
of  feldspar,  mica-gneisses,  if  these  terms  can  be  applied  to  rocks  that  have 
an  essentially  granitic  texture,  although  the  original  banding  of  the  sedi- 
ments remains  in  the  recrystallized  rock  and  causes  it  to  break  more  readily 
along  these  bands  than  in  any  other  dir«iCtion. 

The  action  of  the  sills  is  very  slight  upon  the  slates,  in  most  cases 
producing  rocks  which  are  mica-schists,  but  not  so  completely  recrystallized 
as  in  the  case  of  those  affected  by  the  gabbro. 

Endomorphic  action — There  is,  along  the  margin  of  the  gabbro,  a 
somewhat  finer  grained  facies  than  further  in  the  mass.  The  sills  also 
show  distinct  selvages,  but  neither  the  gabbro  nor  the  sills  appear  to  have 
had  their  textures  or  general  characters  otherwise  modified  as  a  result  of 
contact  with  the  various  rocks  mentioned. 


420 


THE  VERMILION  IRON-BEARING  DISTRICT. 


IRON-OXIDE   BODIES  IN   THE  GABBRO. 

It  has  been  known  for  a  long  time  that  there  were  iron-oxide  masses- 
in  northeastern  Minnesota  within  the  area  which  is  underUxin  by  the  great 
Keweenawan  gabbro  mass.  One  of  these,  which  occurs  on  Mayhew  Lake, 
was  seen  and  described  by  Chauvenet  in  1883  and  1884,"  and  since  that 
time  there  have  been  a  number  of  brief  mentions  made  of  these  gabbro 
iron-oxide  masses  in  the  various  papers  on  the  geology  of  Minnesota  to 
which  reference  has  been  made  in  this  chapter.  The  iron  oxide  occurs  in 
bodies  of  varying  size  and  the  existence  of  these  bodies  has  aroused  a 
considerable  interest  among  investors. 

CHARACTER,    OCCURRENCE,    AND    ORIGIN    OF    THE    IRON-OXIDE    BODIES. 

These  bodies  consist  of  titaniferous  magnetites,  and  have,  according  to 
H.  V.  Winchell,*  the  following  composition: 

Analyses  of  iron-oxide  bodies. 


Constituents. 


Silica,  SiO, 

Alumina,  AUOj , 

Titanium  binoxide,  TiO™ 

Magnetic  oxide  of  iron 

Protoxide  of  iron 

Chromium  sesquioxide,  CrO 

Magnesia,  MgO 

Lime,  CaO 

Phosphoric  acid 


2.02 

2.68 

12.09 

80.  78 


2.40 


Metallic  iron. 


Tr. 
.03 


100. 00 

58.48 


II. 


20.90 
1.75 
2.23 

70.29 
2.01 


2.63 

Tr. 

None. 


99.81 
52. 46 


In  view  of  the  fact  that  a  great  deal  of  money  has  been  spent  in  this 
district  in  exploring  for  nickel  ore,  which  has  been  reported  as  occurring  in 
the  titaniferous  magnetites  in  the  gabbro,  the  following-  statement  by  Grant 
is  valuable: 

Nickel  does  occur  in  similar  situations,''  and  is  frequently  found  as  an  impurity 
ill  one  of  the  gabbro  minerals  (olivine),  but  as  yet  no  reliable  reports  of  nickel  ore 

"  W.  M.  Chauvenet,  U.  S.  Geol.  Survey,  manuscript  notes. 
''Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Bull.  No.  6,  1891,  p.  141. 

'■This  evidently  has  reference  to  its  occurrence  in  other  districts;  not  in  Minnesota,  however. 
Clements. 


THE  KEWEENAWAN.  421 

rich  enough  for  mining-  have  been  received.  A  determination  of  the  niekel  in  iron 
ore  from  the  locality  of  the  specimens  above  analyzed  showed  less  than  one-half  of 
one  per  cent/' 

The  iron-oxide  bodies  occur  in  the  midst  of  massive  granular  normal 
gabbros,  and  are  not  separated  from  the  gabbro  by  sharp  lines.  The  titan- 
iferous  magnetite  body  on  the  margin  of  any  outcrop  next  the  gabbro  is 
lean,  and  has  a  large  projjortion  of  pyroxene,  olivine,  and  feldspar,  in 
general  of  gabbro  minerals,  mixed  with  the  titaniferous  magnetite.  These 
minerals  become  fewer  toward  the  main  magnetite  mass,  but  in  the  direction 
of  the  gabbro  they  increase  in  quantity,  first  giving  varieties  of  highly 
mtignetitic  gabbro,  but  gradually  passing-  into  the  crystalline  gabbro,  with 
black  and  gray  mottlings.  There  is  then  a  transition  from  the  gabbro  into 
the  titaniferous  magnetite  bodies,  which  are  but  very  magnetitic  portions  of 
the  gabbro. 

The  high  percentage  of  titanium  in  the  magnetite  bodies,  as  shown  by 
the  above  analyses,  is  very  important  from  both  the  economic  and  the 
scientific  standpoint.  In  the  first  place,  the  titanium  renders  the  magnetite 
at  present  valueless,  since,  in  the  present  iron-smelting  practice,  titaniferous 
ore  can  not  be  smelted  economically.  The  magnetites  can,  therefore,  not 
compete  now  with  the  cheap  nontitaniferous  ores,  nor  can  they  in  the 
future,  unless  new  discoveries  give  a  higher  value  to  titaniferous  ores,  or 
cause  changes  in  iron  smelting  which  will  place  the  titaniferous  ores  on  an 
even  basis  vsdth  the  other  iron  ores. 

The  injurious  effect  of  the  titanium  in  rendering  the  magnetite  unmar- 
ketable would,  of  course,  apply  to  these  titaniferous  magnetite  bodies,  what- 
ever their  size.  However,  so  far  as  we  know,  up  to  the  present  time  no 
published  description  has  been  given  of  any  large  continuous  masses  of  the 
practically  pure  titaniferous  magnetite. 

The  content  of  titanium  is  of  interest  from  a  scientific  standpoint,  in  that 
it  gives  evidence  (additional  to  that  offered  by  the  occurrence)  of  the  intimate 
connection  between  the  gabbro  and  the  ore,  and  enables  us  to  determine  its 
source.  The  gabbro  contains  everywhere  titaniferous  magnetite  in  small 
quantities,  and  the  large  amount  collected  in  these  magnetite  bodies  owes 
its  accumulation  to  those  little  understood  processes  generally  spoken  of  as 
processes  of  segregation.     As  the  result  of  these  processes  the  titaniferoiis 

aLoc.  cit.,  p.  62. 


422  THE  VERMILION  IRON-BEARING  DISTRICT. 

magnetite  elsewhere  widely  disseminated  in  small  quantities  through  the 
gabbro  was  locally  concentrated,  while  the  magma  was  in  a  more  or  less 
fliiid  state.  We  conclude  that  these  titaniferous  magnetite  bodies  belong  to 
those  ii'on-ore  deposits  of  igneous  origin,  which  are  at  present  of  little  value. 
All  of  the  other  ore  deposits  of  this  distiict,  even  when  now  magnetitic,  as 
in  tlie  case  of  the  Gunflint  ores,  are  nontitaniferous,  or  contain  only  traces 
of  titanium,  and  were  originally  of  sedimentary  origin. 

SECTION  II.— ACID  DIKES  YOUNGER  THAN  THE  DLTIilTTH  GABBRO. 

The  great  Keweenawan  gabbro  mass  is  cut  through  at  various  places 
by  small  dikes  of  red  granite.  Several  of  these  were  seen  beyond  the 
limits  of  the  Vermilion  district.  One  especially  was  noted  upon  the  island 
on  the  east  side  of  Cross  River,  just  opposite  the  bay  into  which  the  portage 
from  Snipe  Lake  enters.  This  dike  is  3  feet  in  width  and  trends  north  and 
south.  It  is  a  fi-esh  biotite-granite.  Grrant"  reports  a  granite  dike  cutting 
thi'ough  the  gabbro  on  an  island  in  Gobbemichigamma  Lake  in  the  NW.  ^ 
of  the  NE.  ^  of  sec.  6,  T.  64  N.,  R.  5  W.  These  occun-ences  are  sufficient 
to  prove  that  there  is  a  g-rauite  later  than  the  gabbro. 

South  of  the  Vermilion  district  there  are  large  areas  underlain  by  a 
bright-red  weathering  acid  rock,  varying  from  syenite  to  granite,  which  sends 
off  shoots  into  the  adjacent  gabbro.  No  attempt  has  been  made  to  trace 
the  connection  between  the  granite  dikes  mentioned  above  and  these  lai'ger 
masses  of  "red  rock"  occurring  in  the  midst  of  the  Keweenawan.  The 
possibility  of  their  close  relationship  is  suggested,  however. 

SECTION  III.— BASIC  INTRUSIVES  YOUNGER  THAN  THE  DULUTH 

GABBRO. 

At  numerous  places  basic  dikes  have  been  noted  in  the  gabbi'o.  These 
were  found  as  the  result  of  the  limited  studies  made  upon  the  gabbro. 
These  studies  were  confined  chiefly  to  the  margin  of  the  gabbro  and  to  a 
few  excursions  made  within  the  Duluth  gabbro  mass.  Unquestionably  great 
numbers  of  other  dikes  of  similar  character  would  have  been  found  had  the 
gabbro  been  more  closely  examined.  A  number  of  basic  dikes  were  also 
found  in  the  Upper  Huronian  sediments  and  in  the  older  rocks  of  the  Ver- 
milion district,  both  tlie  eruptive  and  sedimentarv  ones.  These  dikes  corre- 
spond in  every  detail  with  those  found  in  the  gabbro.     All  of  these  very 

oGeol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Kept.,  Vol.  IV,  1899,  p.  479. 


THE  KEWEENAW  AN.  423 

fresh  rocks  are  basalts  or  dolerites,  and  are  believed  to  belong  to  the  same 
general  period  of  formation. 

The  distribution  of  these  dikes  can  be  seen  on  the  maps  in  the  accom- 
panying atlas,  where  those  of  sufficient  size  to  warrant  it  have  been  shown. 
The  majority  of  the  dikes  are  small  and  have  been  omitted  in  most  cases, 
although  a  few  have  been  inserted  upon  the  map  but  are  greatly  exaggerated. 
These  dikes  cross  the  strike  of  the  slate  and  other  sediments  and  are  also 
found  to  run  parallel  to  their  bedding.  In  some  of  the  schistose  rocks 
the  dikes  very  clearly  followed  the  schistosity. 

The  dikes  can  not  be  said  to  follow  a  definite  system  in  their  occur- 
rence, although  presumably  if  they  were  studied  in  sufficient  detail  it  would 
be  found  that  they  followed  in  general  the  lines  of  fracture  which  prevail 
in  those  portions  of  the  district  in  which  they  occur. 

PETROGRAPHIC  CHARACTERS. 

Macroscopic  characters. — The  rocks  are  invariably  dark  colored,  black 
or  greenish  or  brownish  black.  The  rocks  in  the  smaller  masses,  the 
narrower  dikes,  show  a  very  fine  grain  and  could  properly  be  called  basalt. 
The  rocks  in  the  centers  of  the  larger  dikes  show  up  as  coarse-grained 
ophitic  textured  rocks  and  are  dolerites.  However,  gradations  between  the 
fine-  and  coarse-grained  forms  are  shown  in  that  the  finer-grained  phases 
appear  upon  the  margins  of  the  large  dikes  grading  up  by  increasing  size  of 
mineral  constituents  to  the  coarse-grained  dolerites,  occupying  the  centers 
of  the  large  dikes.     The  fine-grained  basalts  seem  to  predominate. 

Microscopic  characters. — Under  the  microscope,  the  rocks  are  found 
to  be  very  fresh,  as  one  would  infer  fi-om  the  fact  that  they  are  usually 
decidedly  black.  In  some  few  of  them  slight  alterations  producing  greenish 
or  brownish-green  minerals  tend  to  vary  these  to  the  brown  or  greenish 
tones  already  referred  to.  The  constituents  of  the  rock  are  yellowish  to 
violet  augite,  green  hornblende,  a  feldspar  near  labradorite  in  composi- 
tion, olivine,  apatite,  ilmenite,  and  magnetite.  These  minerals  show  their 
normal  characters  and  but  rarely  give  evidence  of  being  altered.  Where 
altered  there  has  been  produced  chlorite,  epidote,  calcite,  and  hornblende. 
The  ophitic  texture  predominates  in  these  rocks,  although  the  intersertal 
texture  is  also  common  and  merges  at  places  into  an  imperfect  fluidal 
texture  brought  out  by  the  parallel  arrangement  of  the  feldspars.  Some  of 
the  rocks  are  porphyritic. 


424  THE  VERMILION  IRON-BEARING  DISTRICT. 

RELATIONS  TO  ADJACENT  FORMATIONS  AND  AGE. 

These  basic  rocks  wherever  found  are  clearly  intnisive  in  the  rocks 
which  surround  them.  Hence,  in  all  cases  the  rocks  are  younger  than  those 
in  which  they  occur.  These  dikes  cut  all  the  rocks  of  the  district  from  the 
Archean  up  to  and  including  the  Keweenawan  gabbro,  but  have  not  thus 
far  been  found  cutting  the  granite  dikes  which  were  found  in  the  gabbro  in 
the  Vermilion  district.  However,  in  the  Keweenawan  area  south  of  the 
Vermilion  district  similar  basalt  dikes  have  been  seen  cutting  through  the 
syenite  and  granite  massives  referred  to  above,  and  it  is  strongly  probable 
that  these  dolerite  dikes  are  of  the  same  age.  Since,  then,  these  intrusives 
cut  all  other  rocks  of  the  district  they  must  be  the  youngest  of  the  pre- 
Cambrian  rocks  which  are  represented. 

METAMORPHIC  EFFECTS. 

In  several  instances  where  the  contacts  between  the  Rove  slates  and 
the  dikes  were  exposed,  observation  showed  that  the  slates  were  indurated 
in  the  vicinity  of  the  dikes,  this  induration  diminishing  as  the  distance 
from  the  dikes  increased.  Special  observations  would  be  required  to  deter- 
mine accurately  the  character  of  the  metamorphism  which  produces  the 
induration.  These  examinations  were  not  waiTanted  by  the  amount  of  time 
available  for  the  study  of  the  district,  consequently  the  details  of  this  change 
must  be  left  for  future  studies.  In  all  probability  this  change  consists  in 
the  recrystallization  of  the  quartz  and  feldspar  and  the  production  of  biotite, 
as  is  found  to  be  the  case  in  the  contacts  of  the  Logan  sills  and  dikes  upon 
the  same  slates. 


CHAPTER   VII. 

THE  DRIFT. 

The  Vermilion  district,  like  all  the  rest  of  the  Lake  Superior  region,  was 
overridden  by  the  great  ice  sheets  of  Glacial  time.  The  tendency  of  the 
glacial  action  during  the  advance  of  the  ice  sheet  was,  of  course,  to  reduce 
this  district  to  a  general  level,  to  round  the  hills,  and  produce  strise  upon 
the  rock  surfaces,  thus  marking  the  direction  of  its  movement.  In  its  retreat 
this  process  of  leveling  was  continued  by  the  filling  in  of  the  pre-Glacial 
valleys  with  morainal  deposits.  While  it  is  known  from  the  researches  of 
the  glacialists  that  the  Lake  Superior  region,  and  of  course  that  part  of  it 
here  discussed,  was  covered  several  times  by  ice  sheets,  we  can  recognize 
in  the  Vermilion  district  the  effects  of  the  ice  only  during  the  last  or  Wis- 
consin stage  of  the  Glacial  epoch,  to  which  consequently  belong  all  of  the 
deposits  which  will  be  briefly  described. 

According  to  Prof.  J.  E.  Todd,  the  glacial  deposits  of  northeastern 
Minnesota  can  be  refen-ed  "to  two  great  lobes  of  the  ancient  ice  sheet,  a 
shorter  one  moving  southwest  through  the  Lake  Superior  Basin,  an,d  a  longer 
one  moving  ai'ound  this  from  the  noi^theast  to  the  west  and  southwest."" 

In  order  that  the  reader  may  get  a  clear  idea  of  the  glacial  history  of 
the  Vermilion  district,  it  may  be  well  to  make  some  general  statements 
concerning  the  glacial  history  of  that  portion  of  Minnesota  which  is  adjacent 
to  but  outside  of  the  district  considered  in  this  paper.  Extending-  in  an 
approximately  northeasterly  direction  through  northeastern  Minnesota  there 
is  a  height  of  land  which  forms  the  watershed  between  the  hydrographic 
basins  of  Lake  Superior  on  the  south,  belonging  to  the  continental  St. 
Lawrence  basin,  and  between  the  basin  of  the  Rainy  River  and  Lake  of  the 
Woods,  wliich  belongs  to  the  great  Hudson  Bay  basin  on  the  north.     This 

a  A  revision  of  the  moraines  of  Minnesota,  by  J.  E.  Todd:  Am.  Geol.,  Vol.  XVIII,  1896,  pp. 
22.5-226,  a  paper  read  at  the  August  meeting  of  the  American  Association  for  the  Advancement  of 
Science,  before  Section  E,  Geology  and  Geography.  Also,  Am.  Jour.  Sci.,  4th  ser.,  Vol.  VI,  1898, 
p.  473. 

425 


426  THE  VEKMILION  IRON-BEARING  DISTRICT. 

heig-lit  of  land  may  be  subdivided  into  several  minor  ridges  following  the 
general  trend  indicated.  Dui-ing  its  greatest  advance  the  Lake  Superior 
lobe  and  the  ice  lobe  to  the  north,  called  by  Elftman"  the  Rainy  lobe,  were 
confluent.  As  the  ice  receded  these  lobes  became  separated,  their -separa- 
tion being  determined  by  the  high  land  just  mentioned  as  lying  north  of 
Lake  Superior.  For  a  time  they  were  close  together,  forming  an  inter- 
lobular moraine.  As  they  receded  and  became  more  widely  separated,  each 
formed  independent  moraines.'  Between  these  moraines  there  occurred 
V-shaj)ed  areas,  with  the  apex  of  the  V  pointing  to  the  northeast,  the  arms 
becoming  more  widely"  separated  as  they  are  followed  to  the  west. 

No  deposits  of  the  Lake  Superior  lobe  are  known  in  the  Vermilion 
district.  There  has  been  recognized,  however,  and  described  by  Upham," 
a  great  moraine  deposited  by  the  Rainy  lobe,  which  has  been  named  by 
him  the  Vermilion  moraine.  Moreover,  the  records  of  strise  collected  by 
Upham  "*  and  inserted  on  his  map  in  the  same  article  show  the  direction  of 
the  ice  flow  to  have  been  in  the  main  to  the  south-southwest,  varying  from 
S.  10°  to  S.  50°  W.,  and  seem  to  corroborate  Todd's  division  of  the  ice  sheet 
in  this  region  into  the  two  lobes  as  mentioned  above.  Since  Upham's  work 
this  Vermilion  moraine  has  been  further  described  and  more  accurately 
delimited  by  Elftman.'  No  special  study  of  the  glacial  deposits  of  the 
Vermilion  district  has  been  made  by  the  United  Spates  geologists,  as  the 
work  in  this  district  was  primarily  undertaken  with  the  object  of  deter- 
mining the  pre-Glacial  geology.  Nevertheless,  observations  made  in  the 
course  of  the  survey  enable  us  to  add  a  little  to  the  knowledge  of  the  course 
in  detail  of  this  moraine.  These  observations  have  been  made  use  of,  and 
accordingly  the  moraine  has  been  traced  as  is  shown  on  the  accompanying 
map  (fig.  23.  At  the  time  of  the  formation  of  the  moraine  the  ice  in  the 
northeastern  portion  of  the  district  passed  over  this  east  end  of  the  Giants 
range,  and  the  moraine  was  deposited  to  the  south  of  it.     As  we  follow 

«  The  geology  of  the  Keweenawan  area  in  northeastern  Minnesota,  by  A.  H.  Elftman:  Am.  Geol., 
Vol.  XXI,  Feb.,  1898,  p.  108. 

ftTodd,  op.  cit.  Am.  Jour.  Sci.,  4th  ser..  Vol.  VI,  1898,  p.  473;  Elftman,  op.  cit.,  pp.  90-108. 

'■  Preliminary  report  of  field  work  during  1893  in  northeastern  Minnesota,  chiefl>'  relating  to  the 
glacial  drift,  by  Warren  Upham:  Geol.  Nat.  Hist.  Survey  of  Minnesota,  Twenty-second  Ann.  Rept., 
1893,  pp.  18-66. 

''  Upham,  op.  cit.,  pp.  38—40. 

f  Elftman,  op.  cit,  p.  94. 


THE  DRIFT. 


427 


the  moraine  westward  we  find, 
however,  that  it  gradually  ap- 
proaches the  Giants  range, 
which  has  a  trend  to  the  south 
of  west,  and  crosses  it  in  T.  63 
N.,  R.  9  W.  West  of  this  point 
the  ice  tongue  reached  a  point 
considerably  farther  south,  and 
it  had  sufficient  strength  to  ad- 
vance well  up  the  slopes  of  the 
range,  but  was  never  able  to 
surmount  it.  From  here  on 
the  moraine  is  found  lying  first 
high  up  upon  the  northern 
slope  of  the  range,  with  the 
distance  increasing  between  it 
and  the  range  as  we  go  farther 
west.  In  the  western  portion 
of  the  district  it  is  nearly  15 
miles  north  of  the  range. 

Exceptionally  heavy  deposits 
of  drift  are  known  to  occur 
in  the  following  areas:  Sees. 
21-28,  T.  61  N.,  R.  15  W.,  and 
extending'  from  there  on  up  to 
the  northeast  in  a  belt  approx- 
imately 2  miles  wide  through 
sees.  19,  30,  18,  29,  16,  21,  10, 
15,  22,  2,  11,  14,  1,  12,  T.  61 
N.,  R.  14  W.;  sec.  6,  T.  61  N., 
R.  13  W.;  sees.  14,  15,  22,  23, 
T.  62  N.,  R.  13  W.;  sees.  25,  26, 
34,  35,  36,  T.  63  N.,  R.  12  W.; 
sees.  30,  31,  T.  63  N.,  R.  11  W.; 
sees.  20,  21,  16,  15,  14,  11,  12, 
T.  63  N.,  R.  11  W.;  sees.  3,  4,  6, 


428  THE  A^ERMILION  IRON-BEARINQ  DISTRICT. 

8,  9,  10,  T.  63  N.,  R.  10  W.  The  main  moraine  has  been  traced  by  cou- 
necting  up  these  heavy  diift  dejiosits.  Its  distribution  can  be  seen  in 
lig.  23.  In  the  eastern  portion  of  the  area  the  course  of  the  moraine  is 
essentially  that  as  given  in  Elftman's  paper,  referred  to  above,  with  but  a 
few  minor  changes.  This  main  moraine  is  not,  however,  the  only  moraine 
which  is  present  in  this  district.  There  are  several  other  areas  which  have 
been  observed  in  which  a  clearly  developed  kettle  moraine  topography  is 
present.  Such  a  series  of  areas  have  been  connected  extending  from  near 
the  south  shore  of  Vermilion  Lake  to  the  north  of  east,  running  approxi- 
mately along  the  town  line  between  Ts.  61,  62  N.,  Rs.  14,  1,5  TT.,  and 
then  extending  to  the  northeast  across  Eagle  Nest  lakes  into  the  western 
poi-tion  of  T.  62  N.,  R.  13  W.  This  small  moraine  is  for  the  greater  part  of 
its  extent  nearly  parallel  with  the  main  Vermilion  moraine.  In  the  east, 
however,  it  approaches  the  moraine,  and  finally  coalesces  with  it.  It  is 
evidently  a  moraine  representing  one  of  the  stages  in  the  recession  of  this 
lobe  during  which  this  extreme  southwestern  portion  of  the  lobe  in  the 
lower  land  retreated  relatively  much  more  rapidly  than  did  the  southeast- 
erly margin  of  the  ice.-  Still  another  stage  in  the  recession  is  represented 
by  morainal  deposits  extending  from  north  of  Armstrong  Bay  of  Vermilion 
Lake  northeast  through  the  northern  portion  of  T.  62  N.,  R.  14  W.,  and  the 
southern  portion  of  T.  63  X.,  R.  13  W.,  south  of  Burntside  Lake.  Still 
other  deposits  were  observed  on  Birch  Point  and  on  Pine  Island,  in  Ver- 
milion Lake.  These  could  probably  be  traced  to  the  noii;heast  of  Vermilion 
Lake,  but  no  sjjecial  work  having  been  done  in  this  area  the  continuation 
of  these  deposits  is  not  known.  Other  terminal  moraine  deposits  north  of 
the  main  Vennilion  moraine  are  known  to  occur  in  the  southern  poi-tion  of 
T.  64  N.,  Rs.  9,  10  W.;  in  the  southwestern  portion  of  T.  64  N.,  R.  9  W., 
north  of  Moose  and  Newfound  lakes;  and  in  the  southern  part  of  T.  66  N., 
Rs.  5  and  6  W.,  to  the  northwest  of  West  Grull  Lake. 

Over  the  remainder  of  the  district  the  di-ift  is  comparatively  thin,  and 
in  fact  in  places  only  a  few  bowlders  on  the  tops  of  the  hills  with  occasional 
strife  and  the  rounded  outline  of  the  hills  indicate  the  former  presence  of  the 
ice  sheet.  The  thinness  of  this  didft  is  especiallv  noticeable  in  the  eastern 
l)art  of  the  district.  In  the  western  portion  of  the  district  the  general  di-ift 
mantle  is  much  thicker  than  it  is  in  the  eastern  poi'tion.  The  general  effect 
of  this  drift  is,  (.)f  course,  to  cover  all  of  the  rocks,  and  in  agreement  with 


THE  DRIFT.  429 

the  distribution  given  above  we  find  that  in  the  western  portion  of  the 
district  exposures  of  rock  in  situ  are  far  less  numerous  on  the  whole  than  in 
the  eastern  part,  where  the  drift  was  apparently  mucli  thinner  to  begin 
with,  and  where  since  the  original  jDre-Glacial  relief  was  more  marked,  it 
has  to  a  considerable  extent  been  removed  from  the  crests  of  the  hills  by 
post-Glacial  erosion,  which  is  there  correspondingly  more  vigorous. 

The  width  of  the  Vermilion  moraine  proper  can  not  be  given  with  any 
very  great  degree  of  accuracy.  It  varies  considerably,  ranging  from  a 
half  mile,  and  perhaps  even  less,  up  to  between  2  and  3  miles  in  T.  61  N., 
R.  14  W. 

The  depth  of  the  drift  constituting  the  moraine  is  also  variable. 
Test  pits  and  drill  holes  have  cat  through  it  in  a  number  of  places  to  a 
depth  of  75  feet.  It  is,  of  course,  in  many  places  much  thinner  than  this. 
Judging  from  the  very  considerable  irregularities  noticed — for  instance,  in 
the  area  southwest  of  Eagle  Nest  lakes,  along  the  township  line  between 
Ts.  61  and  62  N.,  E.  14  W.,  and  in  the  moraine  extending  northeast- 
southwest  across  T.  61  N.,  E.  14  W.,  and  also  in  the  area  southeast  of  Ely — 
it  must  run  up  to  at  least  150  to  200  feet  in  depth,  and  may  even  reach  a 
greater  thickness  than  this. 

Between  the  south  edge  of  the  moraine  and  the  Giants  range,  especi- 
ally in  Ts.  60,  61  N.,  Es.  14,  15,  and  16  W.,  extensive  deposits  of  drift, 
modified  by  the  waters  from  the  edge  of  the  melting  glacier,  are  well  devel- 
oped. To  the  north  of  the  moraines  in  various  places  similar  modified 
drift  is  found,  which  evidently  owes  its  origin  to  the  modification  by  water 
from  the  Eainy  lobe  after  it  had  passed  to  the  north  of  the  Vermilion 
moraine. 

It  will  be  remembered  that  the  drainage  of  this  district,  which  is 
approximately  bounded  on  the  south-southeast  by  the  high  laud  of  the 
Giants  range,  is  to  the  northwest.  It  is  to  be  supposed  that  as  the  glacier 
retreated  the  waters  from  its  melting  edge  must  have  been  dammed  between 
this  range  and  the  ice  lobe  to  the  north.  It  seems  highly  probable,  there- 
fore, that  glacial  lakes  of  considerable  size  must  have  been  formed  in  the 
Vermilion  district.  A  study  of  the  topography  shows  that  there  are  many 
areas  which  must  have  been  favorable  for  the  development  of  such  lakes, 
but  no  definite  evidence,  such  as  lake  beaches  and  clay  deposits,  pointing 
to  the  existence  of  large  glacial  lakes,  has  been  found.     It  seems  highly 


430  THE  VERMILION  IRON-BEARING  DISTRICT. 

probable,  however,  that  such  lakes  existed  on  the  site  of  what  is  uow  Ver- 
milion Lake,  Basswood  Lake,  and  Sagauaga  Lake,  and  probably  in  many 
other  places.  Indeed,  Winchell  and  Grant*  have  reported  terraced  gravels 
around  Long  Lake  and  White  Iron  Lake,  which  give  clear  evidence  of  the 
existence  of  lakes  at  these  places  during  Glacial  time,  when  the  water  was 
considerably  higher  than  the  present  water  level  and  which  consequentlv 
covered  larger  areas  than  those  of  the  present  lakes.  WinchelP  has  recenth-, 
since  the  above  was  written,  given  a  brief  description  of  the  glacial  lakes 
which  occur  partially  or  wholly  in  the  Vermilion  district. 

According  to  him.  Lake  Annamani  covered  the  present  site  of  Vermil- 
ion Lake,  being  10  or  15  feet  higher  than  it  is.  Lake  Norwood  was  south 
of  the  present  Vermilion  Lake,  and  covered  the  low  flat  area  north  of  the 
Giants  range  along  the  Embarrass  River. 


«Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  Final  Rept.,  Vol.  IV,  1899,  p.  235. 
''Glacial  lakes  of  Minnesota,  by  N.  H.  Winchell:  Bull.  Geol.  Soc.  America,  Vol.  XII,  1901,  pp. 
125-126. 


CHAPTER  VIII. 

TOPOGRAPHY   OF   THE    DISTRICT   IN   ITS   RELATION   TO 
GEOLOGIC   STRUCTURE. 

The  velationsliip  of  topographic  rehef  to  geologic  structural  features 
is  251'ohably  nowhere  better  brought  out  than  in  the  Vermilion  district. 
From  the  preceding  pages  the  reader  will  have  learned  of  the  gabbro 
plateau  (pp.  37,  399)  with  its  simple  topographic  features  due  chiefly  to  the 
homogeneous  character  of  the  gabbro,  the  country  rock,  and  of  the  peculiar 
topography  of  the  sawtooth  hills  area  (pp.  38,  391,  400)  due  to  the  presence 
of  the  sills  intercalated  between  the  beds  of  slates  with  their  monoclinal 
dip  to  the  south-southeast.  The  descriptions  which  follow  apply  especially 
to  the  broad  area  north-northwest  of  the  Giants  range  and  to  the  Giants 
range  itself 

In  these  portions  of  the  Vermilion  district  the  rocks  are  closely  folded 
into  synclines  and  anticlines,  frequently  ai-ranged  en  echelon,  which,  in 
general,  have  an  east-northeast  trend.  Moreover,  the  oldest  rocks  are  the 
hardest,  and  these  usually  occupy  the  anticlines;  whereas  the  troughs  are 
occupied  by  the  younger,  softer  rocks.  The  minor  structures,  such  as 
cleavage  and  fissility,  are  in  general  parallel  with  the  above  structural 
arrangement.  As  a  consequence  of  the  combination  of  these  factors,  which 
are  of  the  greatest  importance  in  determining  the  topography,  we  find  the 
main  ridges  to  be  usually  anticlines  of  older  rock,  with  the  intervening 
valleys  in  synclines  of  younger  and  less  resistant  rocks.  Almost  withoiit 
exception  the  main  ridges  and  valleys,  and  to  a  very  great  extent  the  minor 
ones  also,  agree  with  the  trend  of  the  geologic  structure  and  run  east- 
northeast. 

Beautiful  examples  of  the  relationship  between  the  topography  and 
geologic  structure  are  shown  in  the  cases  of  Tower  and  Lee  hills  at  Tower 
and  of   Soudan   Hill    at   Soudan.     Here    the  anticlinal    hills  of  resistant 

431 


432  THE  VERMILION  IKON-BEARING  DISTRICT. 

Archean  jasper  are  surrounded  by  valleys  in  thfe  softer  rocks  of  Lower 
Huronian  age.  The  numerous  anticlinal  hills  of  Archean  greenstone 
between  Moose  Lake  and  the  Kawishiwi  River,  in  Ts.  63  and  64  K.,  R.  9  W., 
and  those  between  Knife  Lake  and  Gobbemichigamraa  Lake,  in  Ts.  64 
and  65  N.,  Rs.  6  and  7  W.,  show  similar  relationships,  being  surrounded 
by  sedimentaries  of  Lower  Huronian  age.  Somewhat  less  characteristic 
are  the  numerous  small  anticlines  shown  on  the  islands  of  Vermilion  Lake 
and  the  adjacent  shores,  with  Ely  Island,  the  jioint  south  of  Mud  Creek 
Bay,  and  the  point  east  of  Stuntz  Bay  as  the  most  striking  cases.  Just 
across  the  international  boundary  in  Ontario  there  is  an  area  reconnoitered 
by  the  United  States  geologists  where  a  great  Archean  anticline  is  found 
separating  the  Lower  Huronian  Knife  Lake  slates  from  the  Archean  iron- 
bearing  Soudan  formation  in  Emerald  and  Big  Rock  lakes,  and  another 
Archean  anticline  separates  the  iron  formation  of  these  last  two  lakes  from 
the  Lower  Huronian  syncline  in  That  Mans,  This  Mans,  the  Other  Mans, 
and  Agawa  lakes.  These  four  narrow,  aligned  lakes  are  the  most  striking 
cases  of  synclinal  lakes  found  in  this  region. 

The  shapes  of  the  lakes  depend  also  to  a  very  great  extent  upon  the 
structural  features  of  the  rocks  surrounding  the  lakes.  They  lie,  as  has 
already  been  intimated,  in  structural  basins  which  are  occupied  by  the 
younger  rocks.  Examining  the  large  lakes  in  detail  one  finds  that  the 
prominent  salients  are  formed  by  the  anticlines  of  older  rocks,  while  the 
reentrants  are  synclines  occupied  by  the  younger  ones.  This  condition  is 
very  noticeable  on  the  east  end  of  Vermilion  Lake.  Beginning  at  the  south 
with  Stuntz  Bay  we  find  the  east  side  of  this  bay,  which  continues  to  the 
east  in  a  mai'ked  depression,  is  in  a  syncline  of  Lower  Huronian  sediments 
with  an  anticline  of  Archean  greenstone  and  jasper  forming-  Soudan  Hill  to 
the  south,  and  a  corresponding  anticline  of  Archean  jasper  to  the  north, 
which  forms  a  salient.  The  western  side  of  the  bay  shows  clearly  the 
relation  between  the  differential  erosion  of  rocks  of  the  same  series.  It  lies 
in  a  syncline  of  Lower  Huronian  slates  with  the  underlying-  conglomerate 
of  the  same  series  forming-  the  north  arm,  and  Archean  jasper  with  occa- 
sional patches  of  unremoved  Lower  Huronian  conglomerate  forming  the 
south  arm.  Armstrong  Creek  and  Bay  is  another  case  of  a  syncline  occu- 
pied by  the  Lower  Huronian  slates  with  Lower  Huronian  conglomerates  on 
both  flanks,  and  forming  ridges      Still  farther  south  and  north  of  the  con- 


RELATION  OF  TOPOGRAPHY  TO  STRUCTURE.  433 

glomerate  come  anticlines  of  Arcliean  i-ocks.  Mud  Creek,  about  a  mile  and 
a  half  north  of  Armstrong  Bay,  flows  also  in  a  depression  occupied  by  the 
Lower  Huronian  sediments,  which  are  flanked  by  Archean  greenstone. 
Similar  cases  are  shown  in  the  synclines  of  sediments  extending  from  Ver- 
milion eastward  to  Bass  Lake,  in  sec.  2,  T.  62  N.,  R.  15  W.,  and  extending 
eastward  along  Rice  Bay  of  Vermihon  Lake,  in  sees.  34  and  35,  T.  63  N., 
R.  15  W.,  still  farther  north. 

In  T.  64  N.,  R.  9  W.,  Moose  Lake  occurs,  lying  in  a  syncline  of  Lower 
Huronian  slates.  On  the  west  side  of  the  northern  bay  of  Moose  Lake, 
that  one  out  of  which  the  portage  to  Wind  Lake  goes,  the  salient  on  the 
west  side  is  formed  by  an  Archean  greenstone  anticline  with  reentrants  on 
both  sides  in  the  Lower  Huronian  slates.  Knife  Lake  shows  a  beautiful 
case  in  the  2-mile  long  point  of  Archean  greenstone  projecting  west 
through  sees.  22  and  23,  T.  66  N.,  R.  7  W.  Both  to  the  north  and  south  of 
this  erosion  has  very  nearly  removed  all  of  the  Lower  Huronian  sedi- 
ments from  contact  with  this  anticline,  small  patches  only  being  found  on 
the  north  and  south  flanks,  as  shown  on  the  geologic  maps  in  the  accom- 
panying atlas.  Lake  Gobbemichigamma,  in  sees.  31-32,  T.  65  N.,  R.  5  W., 
and  sec.  6,  T.  64  N.,  R.  5  W.,  and  sec.  1,  T.  64  N.,  R.  6  W.,  lies  right  at  the 
junction  of  three  great  formations  and  is  influenced  in  its  shape  very 
markedly  by  them.  On  the  shores  each  formation  shows  the  topographic 
forms  characteristic  of  it.  The  formations  meeting  here  are  the  Archean 
greenstone  and  the  Lower  Huronian  Knife  Lake  slates,  which  overlap 
upon  the  greenstone,  and  in  contact  with  both  of  the  above  and  over- 
lapping them  is  the  Keweenawan  gabbro.  The  Twin  Peaks  Archean 
anticline  forms  a  bold  east-west  ridge,  the  end  of  which  overlooks  the 
lake  and  projects  into  it  from  the  west,  forming  two  salients.  The  sligdit 
embayment  to  the  north  of  this  greenstone  is  along  the  contact  of  the  Archean 
and  the  Lower  Huronian  slates,  while  the  embaj'inent  to  the  south  follows 
a  little  to  the  south  of  the  contact  of  the  Archean  greenstone  and  of  the 
Keweenawan  gabbro.  The  greenstone  runs  down  in  a  very  steep  slope  to 
the  water.  The  north  side  of  the  lake  is  bounded  by  the  slates,  and  here 
there  is  a  gentle  slope  down  to  lake  level.  On  the  southeast  and  south  sides 
of  the  lake  the  gabbro  appears  in  steep  cliffs  and  prominent  headlands,  the 
irregularities  of  the  shore  being  influenced  in  their  trend  by  the  marked 
jointing  of  the  gabbro. 

MON  XLV — 03 28 


434  THE  VERMILION  IRON-BEARING  DISTRICT. 

The  shapes  of  many  of  the  lakes,  especially  those  which  lie  com- 
pletely in  rocks  of  more  or  less  uniform  character,  have  been  determined 
by  the  direction  of  the  joint  planes  of  the  district.  The  major  joints  agree 
very  closely  with  the  direction  of  the  predominant  schistosity  and  trend 
N.  60-80°  E.  The  next  important  system  of  joints  is  approximately  at 
right  angles  to  that  direction.  The  effect  of  this  jointing  can  be  seen  best 
in  the  lakes  lying  within  the  well-jointed  Knife  Lake  slates.  Knife  Lake 
itself  shows  its  dependence  upon  the  jointing  of  the  rocks  by  its  principal 
direction,  which  agrees  with  the  jointing,  and  by  the  occasional  arms 
nearly  at  right  angles  to  it.  The  main  southeast  arm  of  Knife  Lake  owes 
its  direction,  as  can  be  readily  seen  by  reference  to  the  maps  in  the  atlas, 
to  the  influence  of  the  Archean  anticline  which  forms  its  north  shore.  The 
lakes  between  the  east  end  of  Knife  Lake  and  Saganaga  Lake  are  within 
the  area  wherein  the  influence  of  the  anticline  formed  partly  of  the  granite 
of  Saganaga  Lake  makes  itself  felt,  and  where  as  a  result  of  this  the  bed- 
ding of  the  slates,  as  \ve\l  as  the  jointing,  which  agrees  fairly  well  with  the 
direction  of  bedding,  turns  strongly  to  the  east  of  north,  getting  more  nearly 
north,  and  finally  turning  a  little  to  the  west  of  north  as  the  northern  portion 
of  the  granite  mass  is  approached.  This  change  in  direction  of  the  structure 
in  the  slates  and  the  relationship  of  the  extension  of  the  lakes  thereto  is 
shown  by  the  string  of  lakes  in  sees.  26,  34,  and  35,  T.  66  N.,  R.  6  ^Y.,  and 
sees.  3  and  10,  T.  65  N  ,  R.  6  W.  This  string  of  lakes  connecting  the  east  end 
of  Otter  Track  Lake  and  the  east  end  of  the  south  arm  of  Knife  Lake  has 
a  trend  in  general  of  N.  20°  E.  Another  specific  instance  of  the  influence 
of  the  jointing  in  these  slates  can  be  seen  in  the  lakes  in  sec.  30,  T.  66  N., 
R.  5  W.,  and  sees.  24-25,  T.  66  N.,  R.  6  W.,  just  west  of  the  boundaiy 
between  the  Knife  I^ake  slates  and  the  granite  of  Saganaga  Lake.  Here 
the  jointing  and  the  bedding  have  turned  to  about  N.  10°  W.  These  two 
lakes  have  their  major  direction  following  this  line  of  major  jointing.  A 
second  system  runs  N.  25°  E.  and  controls  the  trend  of  the  ends  of  some  of 
the  points  and  the  greatest  width  of  the  lakes. 

Another  factor  which  has  in  many  cases  determined  the  position  of  the 
topographic  features  has  been  the  plane  of  contact  between  the  difterent 
formations  and  the  differential  erosion  of  these  formations.  A  notable  case 
is  that  of  Otter  Track  Lake,  on  the  international  boundary.  This  lake  lies 
along  the  contact  of  the  Archean  greenstone  and  the  Lower  Huronian  Knife 


RELATION  OF  TOPOGRAPHY  TO  STRUCTURE.  435 

Lake  slates.  The  factor  of  differential  erosion  was  aided  here  by  the  strike 
of  the  slates  and  the  jointing,  which  agrees  closely  with  the  strike  of  the  line 
of  contact.  Only  in  one  place  has  any  of  the  Lower  Hnronian  slates  been 
left  upon  the  north  side  of  the  main  arm  of  the  lake.  Before  reaching  the 
east  end  of  this  lake  there  is  a  large  north-trending  bay  which  is  in  Cana- 
dian territory.  The  direction  of  this  bay  has  been  controlled,  like  that  of 
the  main  body  of  the  lake,  by  the  contact  of  the  Lower-  Hnronian  slates 
and  the  Archean  formations  which  here  swings  sharply  to  the  north. 

In  the  western  part  of  the  district,  extending  from  Rice  Bay  of 
Vermilion  Lake,  in  T.  63  N.,  R  15  W.,  over  sec.  36,  T.  64  N.,  R  12  W., 
over  nearly  to  Basswood  Lake,  there  is  an  almost  continuous  line  of  lakes 
and  streams  marking  the  boundary  between  the  Archean  greenstone  and 
those  schists  which  have  been  produced  from  this  greenstone  by  the 
metamorphic  action  of  the  Trout,  Burntside,  and  Basswood  granites  upon  it. 
This  line  begins  near  Rice  Bay,  as  mentioned  above,  and  can  be  followed 
east  through  the  creek  flowing  into  this  bay  from  the  east  nearly  to  the 
vicinity  of  Mud  Lake  in  sec.  3,  T.  62  N.,  R.  14  W.  For  a  few  miles  then 
it  runs  south  of  Burntside  Lake.  From  sec.  32,  T.  63  N.,  R.  13  W.,  it  is 
followed  approximately  by  Burntside  Rivel-  up  to  sec.  24  of  above  township 
and  range.  From  there  on  to  the  east  it  follows  approximately  a  string  of 
lakes — Little  Long  Lake,  Bass  Lake,  and  Mud  Lake.  Another  striking 
instance  of  this  influence  of  the  contact  of  two  rocks  on  the  topography  is 
shown  in  the  case  of  the  Kawishiwi  River.  Beginning  in  sec.  15,  T.  63  N., 
R.  9  W.,  where  it  enters  the  Vermilion  disti-ict,  it  runs  southwest  verv 
closely  along  the  contact  between  the  gabbro  and  the  Lower  Hnronian 
sediments.  Then  when  the  sediments  disappear  it  runs  along  the  contact 
between  the  Archean  greenstone  and  Keweenawan  gabbro,  and  where  the 
Giants  Range  granite  commences  it  follows  the  contact  between  the  gabbro 
and  the  granite.  Shortly  after  the  granite  is  reached  the  Kawishiwi  divides 
in  sec.  26,  T.  63  N.,  R.  10  W.,  the  south  arm  running  southwest  very  closely 
along  the  granite-gabbro  contact.  The  north  arm  of  the  Kawishiwi  continues 
very  nearly  due  west,  following  along  the  contact  between  the  granite  and 
the  schists  produced  by  the  contact  action  of  the  granite  on  the  Archean 
greenstone;  then  between  the  greenstone  and  the  overlying  Lower  Hnronian 
sediments,  and  finally  bends  northwest,  cutting  across  the  greenstone.  In 
the  eastern  part  of  the  district,  in  sees.  34  and  35,  T  65  N.,  R.  5  W.,  a  string 


436  THE  VERMILION  IRON-BEARING  DISTRICT. 

of  lakes  connected  by  a  stream  is  fonnd  along  the  contact  between  tlie  iron 
formation  of  the  Lower  Huronian  and  the  Lower  Hnronian  Og-ishke  con- 
glomerates. A  similar  topographic  depression  can  be  followed  between  the 
contact  of  the  granite  and  the  greenstones  in  sees.  26,  27,  28  and  29,  T.  62 
N.,  R.  13  W.,  and  in  sees  17,  9,  3,  11,  and  1,  T.  62  N.,  R.  12  W.,  to  sec.  31, 
T.  63  N.,  R.  11  W.,  south  of  Ely. 

The  above-mentioned  instances  are  only  some  of  the  more  important 
and  most  noticeable  of  the  many  cases  which  occur  in  the  Vermilion 
district,  and  which  migiit  be  cited  as  illustrating  the  dependence  of  topog- 
raphy upon  geologic  structure.  Some  of  the  other  cases  have  been 
referred  to  in  the  more  detailed  descriptions  of  the  vai'ious  kinds  of  rocks 
and  of  the  special  areas. 


CHAPTER  IX. 

GEOLOGIC  HISTORY  OF  THE  VERMILION  DISTRICT. 

The  earliest  period  o£  the  history  of  this  district,  as  of  all  others,  is 
entirely  hidden  from  ns.  We  can  go  no  farther  back  than  the  time  of  the 
formation  of  the  earliest  rocks  now  exposed  to  view.  Whether  during  this 
early  time  the  area  of  the  Vermilion  district  remained  continuously  xnider 
water  after  the  formation  of  the  primeval  ocean,  or  after  an  ocean  existed 
there  was  a  land  area  antedating  the  development  of  the  oldest  rocks  now 
found,  can  only  be  conjectured.  However  this  may  be,  the  evidence  we 
now  have  indicates  that  this  region  was  for  the  most  part,  if  not  wholly, 
under  water  at  the  time  the  Ely  greenstones  were  formed. 

The  Ely  greenstone,  as  has  been  shown,  consists  entirely  of  igneous 
rocks.  These  are  almost  wholly  lavas  and  largely  of  ellipsoidal  greenstones 
which  are  amygdaloidal  and  spherulitic,  and  are  therefore  presumed  to  be 
surface  igneous  rocks.  These  greenstones  within  the  district  have  a  great 
but  unknown  thickness,  and  the  formation  extends  far  beyond  its  confines 
in  a  continuous  belt  nearly  to  Lake  Nipigon,  Ontario.  Furthermore,  similar 
formations  occupy  equivalent  positions  in  other  parts  of  the  Lake  Superior 
region;  hence  we  infer  that  the  time  of  the  deposition  of  the  Ely  greenstone 
was  one  of  regional  vulcanism  paralleled  in  magnitude  only  by  the  more 
important  later  volcanic  periods,  such  as  those  of  the  Keweenawan  and  the 
Tertiary. 

The  length  of  time  involved  in  the  formation  of  the  Ely  greenstone 
can  only  be  conjectured.  It  is  well  known  that  volcanic  matei'ial  accumu- 
lates with  great  rapidity;  hence,  so  far  as  the  masses  of  material  exposed 
to  view  may  determine  our  judgment,  we  would  not  be  justified  in  con- 
cluding- that  this  period  was  one  of  extraordinary  length.     However,  if  we 

437 


438  THE  VERMILION  IRON-BEAliING  DISTRICT. 

unite  the  events  preceding-  the  time  of  the  Ely  greenstone  with  that  of  the 
greenstone  itself  the  time  thus  represented  would  be  very  long-  indeed. 
This  time  is  only  a  part  of  the  Archean.  So  far  as  the  Vermilion  district 
is  concerned,  and,  indeed,  so  far  as  the  entire  Lake  Superior  region  is 
concerned,  we  get  no  farther  back  than  that  period  of  the  primeval  ocean 
in  which  the  Ely  greenstone  was  formed.  If  there  were  land  areas  in  this 
region  or  elsewhere  in  the  world  at  an  earlier  time  we  have  no  evidence  of  it. 

After  regional  volcanic  activity  continued  for  an  unknown  time,  it 
died  out,  as  volcanic  activit}^  has  elsewhere.  Probably  this  process  was  a 
very  slow  one,  although  we  have  no  direct  evidence  upon  this  point. 

Following  the  Ely  greenstone  there  were  orogenic  movements  and 
erosion.  Correlative  with  these  was  the  deposition  of  sediments  in  the 
Vermilion  district.  The  mechanical  sediments  formed  at  this  time  are 
insignificant  in  amount.  The  depth  of  water  over  the  Vermilion  district  was 
sufficiently  great  to  make  the  mechanical  sediments  entirely  subordinate. 
Here,  under  quiescent  conditions,  the  iron-bearing  carbonate  of  the  Soudan 
formation  was  laid  down.  For  ranch  of  the  district  this  formation  rests 
directly  upon  the  Ely  greenstone,  with  no  intervening  mechanical  material. 
The  iron-bearing  carbonates  are  chemical  or  organic  sediments,  or  were 
more  probably  formed  by  a  combination  of  chemical  and  organic  agents. 

That  life  was  present  in  the  sea  at  the  time  of  the  deposition  of  the 
Soudan  formation  and  furnished  organic  material  to  reduce  the  limonite 
and  carbonate  to  protoxide  is  indicated  by  the  graphitic  material  now 
associated  with  these  rocks.  The  iron  for  the  carbonates  may  have  been 
partly  absorbed  from  the  Ely  greenstone  underlying  the  formation,  but 
probably  was  more  largely  abstracted  from  the  areas  of  Ely  greenstone 
raised  above  the  water  outside  the  district  at  present  considered.  The 
underground  and  surface  streams  would  there  dissolve  the  iron  salts.  Thev 
were  brought  to  the  Vermilion  sea,  and  there  were  probably  precipitated 
as  limonite  and  transformed  to  iron  carbonate  by  processes  previously 
explained. 

Following  the  deposition  of  the  Ely  greenstone  there  was  a  second 
great  outbreak  of  igneous  activity.  At  this  time  various  igneous  rocks 
were  intruded  within  the  Ely  greenstone  and  the  Soudan  ft)rmation.  At 
this  time,  or  earlier,  were  the  great  intrusions  of  granite  represente<l  bv  the 


GEOLOGIC  HISTORY.  439 

Arclieau  acid  intrusives,  the  granites  of  Trout,  Basswood,  Burntside,  and 
Saganaga  lakes  and  connecting-  areas.  The  main  masses  of  these  granites 
are  vast  batholiths,  from  which  there  are  innumerable  oifshoots.  These 
intrusions  cut  the  Ely  greenstone  most  intricatelv,  l)ut  all  of  the  g-ranites — 
for  example,  the  granite  of  Saganaga  Lake — have  not  been  found  in  such 
relations  to  the  Soudan  formation.  It  is  believed,  however,  that  this  age 
relationship  exists  and  that  it  is  only  owing  to  absence  of  the  Soudan 
formation  in  good  development  near  the  granite  of  Saganaga  Lake  that 
dikes  of  the  granite  have  not  been  found  in  it.  This  may,  therefore, 
antedate  the  Soudan  formation  and  reall}^  be  of  early  Archeau  age,  although 
if  this  were  the  case  it  is  thought  probable  that  the  disturbances  attending 
its  intrusion  would  have  raised  the  Ely  greenstone  above  the  sea  at  various 
places  in  the  Vermilion  district,  and  thus  would  have  resulted  in  the 
formation  of  thick  mechanical  sediments  below  the  Soudan  formation.  It  is 
therefore  thought  to  be  more  probable  that  the  granite  is  contemporaneous 
with  the  others,  and  is  later  than  the  Soudan  formation. 

The  Ely  greenstone,  the  meclianical  sediments  preceding  the  Soudan 
jaspers,  the  Soudan  jaspers  themselves,  and  the  intrusive  rocks  following 
the  Soudan  constitute  the  Archean  rocks. 

Probabl}'^  contemporaneous  with  these  great  intrusions  of  igneous  rock ' 
were  the  powerful  orogenic  movements  following  Archean  time.  These 
movements  probably  raised  the  entire  Vermilion  district  above  the  sea. 
They  certainly  folded  the  rocks  into  mountain  masses  of  enormous  extent 
and  exceeding  complexity,  so  that  the  Vermilion  district,  which  up  to  the 
present  time  had  been  a  sea  area,  was  now  a  land  area.  No  sooner  was  the 
district  raised  above  the  sea  than  the  epigene  forces  attacked  the  land  and 
the  process  of  degradation  began.  This  was  very  long  continued,  and  land 
and  sea  erosion  cut  deeply  into  the  previous  formation.  For  larg-e  parts  of 
the  district  it  removed  the  entire  Soudan  formation;  in  places  it  cut  away 
the  Ely  greenstone,  exposing  the  underlying  intrusive  granite. 

Attending  the  granitic  intrusions,  the  orogenic  movements,  and  the 
erosion  were  the  metamorphic  chang-es  in  the  rocks.  Dependent  upon  the 
granite  intn^sion  was  the  deep-seated  alteration  of  the  YAj  greenstone, 
resulting  in  the  production  of  amphibole-schists  and  amphibole-gneisses. 
Adjacent  to  the  granite  masses  the  more  superficial  agents  of  metamorphism, 
and  especially  those  of  weathering,  greatly  changed  the  carbonate  of  the 


440  THE  VERMILION  IRON-BEARING  DISTRICT. 

Soudan  fonnation  to  fen-ugiuous  slates  and  ferruginous  cherts,  although 
residual  iron  carbonate  undoubtedly  remained. 

It  therefore  appears  that  the  batholithic  and  dike  intrusions  of  granite, 
porphyry,  etc.,  and  doubtless  extrusions,  and  the  orogenic  and  profound 
erosion  and  the  metamorphism  were  contemporary  events,  or  at  least  largely 
overlapping.  AVhile  the  intrusive  masses  are  placed  in  the  Archeau  series, 
these  events  chiefly  mark  the  inter- Archean-Huronian  time,  the  most  con- 
spicuous evidence  of  which  is  the  great  unconformity  between  the  Archean 
and  the  Lower  Huronian  series.  It  is  clear,  however,  that  Archean  time  is 
not  sharply  separated  from  inter-Archean-Huronian  time,  but  is  connected 
through  the  intrusive  masses.  If  there  were  volcanics  contemporaneous 
with  the  batholiths  of  granite  and  the  intrusive  masses  of  porphyry,  these 
were  wholly  removed  by  the  great  period  of  erosion  mentioned  below,  since 
no  such  rocks  are  found  in  the  district. 

After  erosion  had  long  continued  either  the  land  was  reduced  to  the 
level  of  the  sea  by  this  process  or  else  subsidence  came,  or  perhaps  erosion 
and  subsidence  combined  to  reduce  the  outer  portion  of  the  Vermilion 
district  to  the  level  of  the  sea.  The  sea  then  encroached  upon  the  land. 
However,  the  land  was  uneven,  with  highlands  here  and  lowlands  there,  so 
that  the  time  of  the  advance  of  the  sea  over  the  district  varied  considerably. 
The  areas  first  encroached  upon  received  a  thick  layer  of  gravel  and 
bowlders,  the  material  being  derived  from  highlands  which  had  not  yet 
been  covei-ed  by  the  sea.  Thus  there  was  built  up  the  great  Ogishke  con- 
glomerate, the  lowest  formation  of  the  Lower  Huronian  series.  When  at 
last  the  sea  had  succeeded  in  entirely  overi'iding  the  district,  the  conditions 
were  no  longer  favorable  for  the  deposition  of  coarse  mechanical  sediments; 
also  it  is  probable  that  a  certain  amount  of  subsidence  further  favored 
quiescent  conditions  at  the  bottom  of  the  sea.  At  this  time  the  Agawa 
formation  was  laid  down  in  certain  favorable  localities,  but  in  small  quan- 
tity, in  the  eastern  half  of  the  district. 

In  the  western  part  of  the  district  no  chemical  or  mechanical  deposit 
was  found  which  may  be  correlated  with  the  Agawa  formation  in  the 
eastern  half  of  the  district.  It  may  be  supposed  that  here  the  water  was 
not  deep  enough  for  such  sediments  to  be  produced. 

Following  the  deposition  of  the  rocks  of  the  Agawa  formation  must 
have  been  a  long-continued  subsidence,  f^r  upon  the  formation  was  laid 


GEOLOGIC  HISTORY.  441 

down  the  great  thickness  of  mechanical  sediments  marked  by  the  Knife 
Lake  slates.  These  were  deposited  as  muds,  grits,  and  fine  gravels.  The 
sea,  therefore,  must  have  been  one  of  moderate  depth;  land  areas  must 
have  been  adjacent,  as  the  formation  is  so  thick  that  a  continual  subsidence 
must  have  occurred  in  order  that  the  g'reat  mass  of  material  could  be  piled 
up.  A  part  of  the  material  was  clearly  derived  from  the  adjacent  land 
areas;  some  portion  was  contributed  by  contemporaneous  volcanoes,  for 
within  the  Knife  Lake  slates  at  various  localities  are  considerable  propor- 
tions of  volcanic  ash,  showing  that  volcanic  ash  was  spread  far  and  wide 
from  volcanic  centers,  and  was  thus  important  for  upbuilding  the  Knife 
Lake  slates.  The  source  of  this  ash  has  not  been  discovered.  The 
Ogishke  conglomerate,  Agawa  formation  and  the  Knife  Lake  slates  together 
constitute  the  sediments  of  the  Lower  Huronian  series. 

Following  the  upbuilding  of  the  sediments  was  another  great  period 
of  igneous  intrusion  marked  by  the  Giants  Range  granite,  the  Snowbank 
granite,  and  the  Cacaquabic  granite.  While  the  acid  intrusives  mentioned 
were  the  dominant  ones,  there  are  certain  more  or  less  schistose  basic  and 
intermediate  intrusives  occurring  in  isolated  dikes  in  the  various  rocks  thus 
far  described  which  belong  to  this  same  general  period  of  igneous  activity. 
The  Snowbank  granite  and  granites  of  equivalent  age,  etc.,  are  placed  with 
the  Lower  Huronian  series,  although  they  more  probably  belong  with  the 
great  geologic  revolution  following  the  compai'atively  quiescent  conditions 
of  Lower  Huronian  sedimentation,  and  contemporaneous  with  lavas  probably 
formed  at  the  surface.  If  such  lavas  were  laid  down  they  were  apparently 
wholly  removed  by  the  deep-seated  erosion  mentioned  below. 

This  revolution  was  caused  by  the  orogenic  movements  which  now 
followed.  These  were  of  the  most  severe  kind,  and  continued  long.  As  a 
result  the  Lower  Huronian  series  were  thrown  into  a  set  of  close  east-west 
folds  and  steeper  cross  folds.  This  folding  must  have  produced  great 
mountain  masses.  No  sooner  did  the  orogenic  movements  raise  the  land 
above  the  sea  than  the  epigene  forces  again  began  their  work  of  hewing  it 
down.  The  period  of  erosion  was  very  long,  so  long  in  fact  that  in  various 
places  the  entire  Lower  Huronian  series  was  cut  through,  laying  bare  the 
Archean.  Contemporaneous  with  and  largely  caused  by  the  folding  and 
partly  by  the  intrusion  of  the  igneous  rocks  and  by  the  erosion  was  a 
second  great  period  of  metamorphism.     The  great  mass  of  muds   of  the 


442  THE  \'ERMILION  IRON-BEARING  DISTRICT. 

Lower  Hnroniau  series  became  transformed  to  slates.  The  sediments 
near  the  Giants  Range  and  Snowbank  granites  were  transformed  to 
schists  and  gneisses.  From  the  iron  carbonates  of  the  Agawa  forma- 
tion ferruginous  slates  and  ferruginous  cherts  were  formed,  although 
this  process  by  no  means  iieared  completion.  It  appears  that  following- 
Lower  Huronian  time,  as  in  inter -Archean-Huronian  time,  igneous  inti-u- 
sions,  orogenie  movements,  erosion,  and  metamorphism  were  largely 
contemporary,  or  at  least  overlapping,  and  that  they  were  largely  the  result 
of  the  same  causes;  that  is  to  say,  there  was  a  time  of  the  readjustment  of 
the  earth's  crust  where  before  there  had  been  a  time  of  sedimentation. 
These  earth  movements  resulted  in  folding  and  in  vulcanism,  and  conse- 
quent upon  this  were  erosion,  and  dependent  upon  all  this  was  metamor- 
phism, both  deep  seated  and  surface. 

Following-  the  unconformity  above  Lower  Huronian  time  the  Vermilion 
district  was  again  depressed  and  the  Upper  Huronian  (Animikie)  series 
was  deposited  above  the  upturned  rocks  of  Archean  and  Lower  Huronian 
age.  Only  a  small  area  in  the  eastern  part  of  the  Vermilion  district  is 
covered  by  the  Upper  Huronian  sediments,  and  we  have  little  data  in  this 
district  for  determining  the  history  of  this  time.  The  basal  formation  of 
the  Upper  Huronian  in  the  Vermilion  district  is  the  Gunflint  formation, 
whose  character  is  such  as  to  indicate  that  it  was  deposited  in  relatively 
deep  water  or  a  protected  area  of  the  sea.  Southwest  of  the  Vermilion 
district  in  the  Mesabi  district  a  conglomerate  occurs  at  the  base  of  the 
Upper  Huronian  below  the  Biwabik  formation  which  is  correlative  with  the 
Gunflint  formation  of  the  Vermilion  district.  We  thus  conclude  that  while 
the  Upper  Huronian  sea  advanced,  conglomerates  were  formed  to  the  west, 
but  that  in  the  Vermilion  district  the  conditions  were  not  favorable  for  the 
production  of  this  kind  of  rock.  Over  the  Giinflint  formation  was  then 
deposited  possibly  10,000  and  more  feet  of  Rove  slates  and  graywackes. 
These  rocks  presumably  covered  the  entire  Vermilion  district  and  probably 
extended  far  northward  beyond  the  district,  but  all  evidence  of  such 
extension  has  been  removed  by  erosion. 

Following  the  period  of  deposition  of  the  Upper  Huronian  there  came 
an  upheaval  succeeded  by  a  long-  period  of  extensive  erosion.  This  erosion 
removed  great  thicknesses  of  the  overlying  Upper  Huronian  rocks  and  in 
some    cases   the    entire    thickness,    and    having    cut    through    the    Upper 


GEOLOGIC  HISTORY.  443 

Huronian  even  cut  down  into  the  Lower  Hnrouian  and  Arcliean  rocks. 
From  evidence  in  other  districts  of  the  Lake  Superior  region,  but  not  from 
evidence  in  the  Vermihon  district,  the  unconformity  represented  by  this 
period  of  erosion  is  known  to  have  been  important. 

Following  the  Upper  Huronian  unconformity  came  the  lava  flows  of 
the  Keweenawan.  These  probably  accumulated  to  immense  thickness 
upon  the  beveled  edges  of  the  Upper  Huronian  series.  There  is  no 
evidence  that  these  flows  were  not  subaerial — that  is,  deposited  upon  the 
Upper  and  Lower  Huronian  and  Archean  series  while  they  were  still  land 
areas.  How  far  the  process  of  upbuilding'  of  the  Keweenawan  had 
continued  before  the  next  great  event  it  is  impossible  to  say,  but  it  is 
probable  that  thousands  of  feet  of  lavas  were  erupted,  and  that  sand- 
stones and  conglomerates  were  deposited  in  the  upper  parts  of  the  series 
between  these  lavas.  If  this  be  the  case,  the  land  must  have  again  sub- 
sided below  the  sea.  However  this  may  be,  after  a  very  considerable 
thickness  of  Keweenawan  lava  had  accumulated,  there  came  the  great 
laccolithic  intrusion  of  Duluth  gabbro,  which  now  has  a  surface  area  of 
nearly  200  square  miles  and  extends  from  Duluth  eastward  beyond  the 
eastern  end  of  the  Vermilion  district,  and  which  bounds  the  Vermilion  dis- 
trict on  the  south  from  the  Kawishiwi  River  eastward.  The  planes  between 
the  various  formations  would  be  the  natural  planes  of  easiest  resistance 
which  an  intrusive  would  follow,  and  these  planes  the  Keweenawan  gabbro 
seems  to  have  followed  for  the  most  part.  In  the  western  and  central 
portion  of  the  district  the  gabbro,  to  judge  from  the  way  in  which  it 
overlaps  on  the  Archean  and  Lower  Huronian  rocks,  and  from  the  fact  that 
it  has  included  in  it  at  various  jjlaces  along  its  northern  edge  masses  of  the 
Gunflint  formation  of  varying  size,  seems  to  have  followed  along  the  plane 
between  the  Upper  Huronian  and  the  Archean  and  Lower  Huronian.  In 
the  east  the  gabbro  laccolith  began  to  rise  and  there  beveled  the  edges  of 
the  Upper  Huronian,  and  at  one  place  is  even  found  intrusive  in  the 
Keweenawan  lava  flows.  It  is  believed  that  the  numerous  sills,  named 
Logan  sills  by  Lawson,  which  are  so  abundantly  found  in  the  Animikie, 
are  but  part  of  the  same  magma  from  which  the  gabbro  came  and  that  they 
were  introduced  at  the  same  time.  Finally  the  great  diabase  dikes  of 
Beaver  Bay  and  other  places  along  the  Minnesota  coast,  which  intruded  the 
Keweenawan  lavas,  are  probably  connected  with  this  great  gabbro  laccolith. 


444  THE  VERMILION  IRON-BEARING  DISTRICT. 

It  is  not  supposed  that  the  intrusion  of  this  enormous  mass  of  lava, 
probably  the  g-reatest  known  laccolith,  occurred  within  a  brief  time.  It 
has  been  noted  that  this  laccolith  is  1.50  miles  long  and  probably  thousands 
of  feet  in  thickness — how  thick  is  entirely  unknown.  The  intrusion  of  so 
vast  a  mass  of  material  must  have  occupied  a  very  great  length  of  time. 
The  parts  earlier  intruded  were  doubtless  solidified  long  before  magma 
ceased  to  enter.  Thus  these  latter  parts  would  be  found  as  dikes  in  the 
earlier  solidified  parts.  There  would  be  great  variation  in  its  coarseness 
of  crystallization.  There  would  be  ample  time  for  the  various  processes  of 
differentiation  by  fractional  crystallization  and  separation  by  gravity  and 
other  23rocesses,  and  thus  is  explained  the  structural  complexity  of  the  gab- 
bro  and  its  great  variation  in  mineral  and  chemical  character.  Perhaps 
contemporary  with  the  intrusion  of  the  gabbro,  perhaps  later  than  it,  perhaps 
in  part  both,  was  the  gentle  tilting  of  the  Keweenawan  lavas,  the  Duluth 
gabbro,  and  the  Animikie,  all  together,  to  the  south,  under  Lake  Superior, 
and  the  much  more  pronounced  northwest  tilting  toward  the  same  lake 
of  the  Penokee  series  south  of  Lake  Superior.  To  this  tilting  in  opposite 
directions  on  opposite  sides  of  the  lake  is  due  the  Lake  Superior  Basin. 

It  is  believed  that  at  the  time  of  the  formation  of  the  Lake  Superior 
syncline  the  Giants  range  anticlinal  area  was  correspondingly  upheaved, 
and  that  thus  the  present  Giants  range  was  actually  created  by  this 
movement,  although,  as  has  been  stated  in  the  previous  pages,  the  location 
of  the  range  was  actually  determined  possibly  as  early  as  the  folding 
following  the  Archean  when  the  protaxis  of  the  range  is  thought  to  have 
been  first  formed.  Since  this  earliest  time  repeated  movements  of 
elevation,  particularly  the  one  at  the  close  of  Lower  Huronian  time, 
succeeded  by  subsidence  and  erosion,  have  followed  along  this  old  line  of 
weakness.  The  actual  present  condition  of  the  range  in  its  minor  features 
is  of  course  due  to  the  erosion  and  then  glacial  deposition  which  have 
occured  subsequent  to  Keweenawan  time  aiid  which  are  briefly  outlined 
in  the  following  jDages 

Contemporaneous  with  and  following  the  intrusion  of  the  Keweenawan 
gabbro  is  the  2:)eculiar  metamorphism  which  marks  the  rocks  of  the  Lower 
Huronian,  Upper  Huronian,  and  Archean  along  its  border.  It  has  been 
noted  that  the  Gunflint  formation  adjacent  to  it  was  changed  to  a  banded 
granitic  textured  rock,  consisting  of  iron  silicates,  magnetite,  and  quartz. 


GEOLOGIC  HISTORY.  445 

These  iron  silicates  comprise  chrysolites,  pyroxenes,  and  amphiboles. 
They  are  very  coarse  grained  and  they  stand  as  the  extreme  of  deep-seated 
static  metamorphism  of  an  iron-bearing  cai'bonate.  The  rands  and  grits  of 
the  Knife  Lake  formation  and  the  conglomerates  of  the  Ogishke  formation 
adjacent  to  the  gabbro  were  completely  crystallized  largely  into  granitic- 
textm-ed  rocks  described  (pp.  315,  342).  These  are  the  best  representatives 
of  the  production  of  granitic  textured  rocks  from  heterogeneous  mechanical 
sediments  known  to  the  writer.  They  were  produced  under  deep-seated 
static  conditions  where  high  temperature  prevailed. 

All  of  the  complex  events  thus  far  described  preceded  Cambrian  time. 
This  history  is  Archeau  and  Algonkian.  In  another  place  it  has  been 
shown  by  Walcott"  that  the  Cambrian  transgression  over  the  North 
American  continent  began  at  the  southeast  and  extended  to  the  northwest, 
and  that  it  continued  through  Lower  and  Middle  Cambrian  time  before  the 
sediments  were  deposited  in  the  Lake  Superior  region.  This  great  erosion 
period  was  probably  partly  contemporaneous  with  the  tilting  which  pro- 
duced the  Lake  Superior  syncline.  The  erosion  of  this  time  laid  bare  the 
great  laccolith  of  gabbro,  as  well  as  tlie  beds  of  the  overlying-  lavas,  and 
thus  exposed  to  light  of  day  all  of  the  great  series  of  rocks — within  the 
Vermilion  district  itself  the  Archean,  the  Lower  Huronian,  and  the  Upper 
Huronian  series;  south  of  these  the  great  batholith  of  gabbro;  and  south 
of  this  the  Keweenawan  lavas.  Finally,  however,  the  sea  overrode  this 
region,  and  the  Cambrian  sandstone  was  laid  down  in  the  Lake  Superior 
Basin  and  along  its  border.  Remnants  of  it  have  been  found  far  inland, 
but  none  in  the  Vermilion  district  itself  However,  it  can  not  be  doubted 
that  over  the  Vermilion  district  were  deposited  Cambrian,  or  Silurian  rocks, 
and  therefore  the  Paleozoic  was  there  represented. 

The  next  great  step  in  the  history  of  the  region  was  the  elevation  of 
the  land  above  the  sea  and  long- continued  denudation.  This  period  of 
denudation  has  generally  been  known  as  the  Cretaceous  period  of  base- 
leveling.  Whether  the  sea  actually  overrode  the  Vermilion  district  and 
there  deposited  the  Cretaceous  rocks  is  uncertain.  However,  it  is  certain 
that  the  Cretaceous  sea  reached  within  a  comparatively  short  distance  of 
the  district,  for  outliers  of  the  Cretaceous  rocks  are  now  known  within  20 

«  Correlation  Papers:  Cambrian,  hy  C.  D.  Walcott:  Bull.  U.  S.  Geol.  Survey  No.  81,  1891,  p.  36-1. 


446  THE  VERMILION  IRON-BEARING  DISTRICT. 

miles  southwest  of  ^'ermilion  Lake,  and  it  seems  liighly  probable  that  Cre- 
taceous rocks  were  laid  over  the  district.  However,  iu  this  case  the  long- 
continued  erosion  of  Cretaceous  time  had  probably  removed  all  of  the 
Paleozoic  sediments,  and  had  reduced  the  land  to  a  rough  peneplain  before 
the  de^josition  of  the  Upper  Cretaceous  rocks. 

It  can  not  be  said  that  this  period  of  base-leveling  in  the  Vermilion 
district  was  nearly  so  complete  as  in  central  Wisconsin.  However,  the 
hills  rise  to  approximately  the  same  altitude.  If  one  ascends  to  some  high 
point  he  finds  an  approximate  horizon  line  above  which  onl}^  a  few  points 
project,  as,  for  instance,  the  Sawtooth  range. 

Following  the  period  of  Cretaceous  base-leveling  the  land  was  raised 
approximately  to  its  ^^resent  altitude,  and  a  second  cycle  of  erosion  was 
inaugurated.  This  cycle  has  continued  to  the  present  time.  In  the  early 
part  of  this  long  cycle  river  erosion  was  the  important  factor,  and  at  this 
time  were  scooped  oiit  the  longitudinal  valleys  following  the  softer  rocks 
and  the  various  structures  of  the  rocks.  The  drainage  was  adjusted  to  the 
character  of  the  rocks.  The  slate  areas  were  largely  valleys;  the  more 
resistant  areas  were  lai'gely  highlands.  Finally,  there  came  the  vai'ious  ice 
advances,  at  which  time  the  valleys  were  widened  and  deepened,  the  hills 
were  rounded,  and  glacial  debris  was  dropped  here  and  there,  but  especially 
in  the  valleys.  When  the  ice  last  receded  the  pi*esent  topography  was  sub- 
stantially shaped.  The  depressions  filled  with  water  until  they  overran 
their  rims  at  the  lowest  points,  thus  forming  the  lakes.  The  lakes  were 
thus  connected  by  streams,  and  the  present  irregular  drainage  (discussed 
on  i)p.  39-46)  was  inaugurated.  Thus  we  have  explained  the  present 
topography  of  the  district. 

In  conclusion,  we  see  that  the  Vermilion  district  presents  one  of  the 
longest  geologic  histories  of  any  region  in  the  world.  Apparently  the  Ely 
greenstone,  the  most  ancient  formation,  was  laid  down  in  primeval  time 
and  the  Soudan  formation  was  deposited  above  it.  Since  that  time  there 
were  five  great  periods  of  deposition:  The  Lower  Huronian,  the  Upper 
Huronian,  the  Keweenawan,  the  Paleozoic,  and  the  Cretaceous. 

There  were  four  great  periods  of  igneous  activity:  The  Ely  greenstone, 
tlie  great  batholithic  intrusions  at  the  end  of  Archean  time,  the  hardl}'  less 
important  batholitliic  intrusions  at  the  end  of  Lower  Huronian  time,  and  the 
great  Keweenawan  period  of  volcanic  extrusion  and  intrusion.     There  was 


GEOLOGIC  HISTORY.  447 

also  possibly  contemporary  volcanic  activity  at  the  time  of  the  Knife  Lake 
slates. 

Finally,  there  were  four  great  periods  of  orogenic  movements, 
denudation,  and  metamorphism :  Following  the  Archean  series;  following 
the  Lower  Huronian;  following  the  Upper  Huronian;  and  following  the 
Keweenawan. 

Also  there  were  three  other  great  periods  of  denudation:  The  Cam- 
brian and  the  Cretaceous  periods  of  base-leveling,  and  finally  the  period 
following  the  Cretaceous,  extending  to  the  present  time. 


INDEX 


.A..  Page. 

Agawa  formation,  age  of --  33,308,330 

characters  of —  24,327-328 

distribution  of  .- 324r-326 

,  exposures  of --  326,330-335 

origin  of -  — 329,440 

petrographic  characters  of 327-328 

relations  of —  303,329-330 

stratigraphic  position  of  ._ ■_.  33,308,329-330 

structure  of 326-327 

thickness  of - 330 

Agawa  Lake,  Agawa  formation  at 325, 329, 330, 331 

Agriculture  of  the  district,  character  of 50 

Ahbe.  F.,  cited  on  mining  methods  in  Vermilion  dis- 
trict  234 

Akeley  Lake,  Gunflint  formation  at -.-  25,109-110 

iron-bearing  rocks  of -  109-110 

Logan  sills  at _ „ 411 

Pewabic  quartzites  at 103-104,110,119 

Akeley  Lake  series,  occurrence  of 119 

Algonkian  period,  geologic  history  of  _ 437-445 

Algonkian  rocks,  components  of 105 

map  showing 32 

Amphibole-schists  derived  from  greenstones,  com- 
position of 157 

Amygdaloidal  greenstones,  occurrence  of 139-141,165 

plate  showing _ 168 

Anderson,  C.  L.,  and  Clark,  Thomas,  report  made 
by,  on  geology  and  geological  survey  of 

Minnesota 66 

Annamani  Lake  (a  glacial  lake),  site  of 430 

Animikie  series,  age  of 106,114,123,124-125,396 

areas  of -__  25,88,375 

character  of -. 25-26, 

74, 88, 96, 97, 102-103, 106, 120-122, 377-387, 392-398 

components  of 25, 33, 88, 102-103, 109, 115, 121, 374 

deposition,  of  conditions  of 442 

dip  of. 88,109 

divisions  of , 25, 33, 88, 102-103, 109, 115, 121, 374 

equivalents  of 80-81,92,118 

.    exposures  of 120-122 

formations  composing 23,  a3, 102-103, 121, 374 

metamorphism  of 25-26, 122. 383-387, 393-394 

relations  of ,  to  adjacent  formations 84, 

86, 88, 93, 96, 97. 105, 109-110, 114-116, 387-390, 395 

toHuronian  quartzite 93 

to  Kewatin  rocks.. 84,88,93,109 

to  Logan  sUls 408-410 

to  Vermillion  slates 86 

stratigraphio  position  of 78, 80-81, 99, 106 

thickness  of 74,390 

See  also  Upper  Huronian. 


MON   XLV — 03- 


-29 


Page. 

Archean  history,  sketch  of 437-439 

Archean  rocks,  character  of 95-96, 99 

components  of 33,99,100,105,116-117,129-130,439 

definition  of... 129-130 

divisions  of . 139-laO 

Ely  gi-eenstone  of 130-173 

granites  of :,.. 124,246-274 

intrusive  rocks  of,  relations  of,  to  Soudan  for- 
mation  199 

relations  of 95,96,281-284 

Soudan  formation  of 172-246 

Archean  history,  sketch  of 437-439 

Area  and  boundaries  of  the  district .32 

Argentiferous  quartz  veins  near  Vermilion  Lake, 

occxirrence  of 67 

Arkose,  occiirrence  of 270 

Armstrong  Bay,  conglomerate  at 287 

Armstrong  Lake,  conglomerate  and  greenstone  at.      195 

rock  banding  near 195 

Augite  of  Cacaquabic  granite,  analysis  of 366 

Am-iferous  quartz  veins  near  Vermilion  Lake,  oc- 
currence of 67 

B. 

fiacon,  D.  H.,  acknowledgments  to 18,30 

cited  on  mining  methods  in  Vermilion  district .      2.34 

Banding  in  Ely  greenstone 149 

Bass  Lake,  slates  at 336 

Basswood  gneiss,  stratigi'aphic  place  of 89 

Basswood  Lake,  elevation  of 34 

granite  at,  age  of 261 

distribution  of ;      258 

exposures  of 258 

folding  in 262 

petrographic  characters  of 259-260 

relations  of 260-261 

topography  of .' 259 

granitoid  and  gneissoid  rocksat 87 

greenstones  near 118 

syenitenear 69 

Upper  Kewatin  rocksat _      120 

Bayley,  W.  S.,  citation  from,  on  Duluth  gabbro..  403,404 
on  iron-bearing  rocks  of  Gunflint  formation.  385 
on  metamorphism  of  slates  by  gabbro  at 

Pigeon  Point,  Minn 394 

on  petrogi-aphy  and  geology  of  Akeley  Lake 

region 103-104 

Bayley,  W.  S.,  geologic  work  in  Vermilion  district 

done  by 17,.30 

Bayley,  W.  S.,  Smyth,  H.  L.,  and  Van  Hise,  C.  R., 

cited  on  origin  of  iron  ores 2.32 

449 


450 


INDEX. 


Page. 

Beaver  Bay  diabase,  occurrence  of 123-124 

Bebb.  E.  C,  work  done  by 17 

Bell,  Eobert,  cited  on  geology  of  the  district 69 

Berkey,  C.  P.,  cited  on  copper  minerals  of  Minne- 
sota  - ---  113,184 

Big  Eock  Lake,  iron  formation  at,  relations  of 210 

jasper  interbanded  with  greenstone  at 19i) 

Bigsby.  John  J.,  geologic  work  in  Lake  Superior  re- 
gion done  by 6't 

Bingoshick  Lake,  Logan  sills  at --      398 

Birch  Lake,  Agawa  formation  at 325 

Pewabic  quartzite near --- 119 

slates  at. - 336,347 

Birch  Point,  glacial  deposits  at - ''28 

Birge,  E.  A.,  cited  on  plankton  of  lakes  of  the  dis- 
trict  - -_    *6 

Biwabik  formation, character  of.. - 377-379 

equivalent  of-.- 25,377-3i9 

thickness  of  -  - 390 

See  also  Pewabic. 

Black  Eiver  schists,  equivalent  of  .- 92 

Bois  Blanc.    See  Basswood. 

Bone  Lake,  Michigan, ci-ystalline  schists  at  -- 198 

Breociation,  examples  of 194-195 

Brule  Lake,  Duluth  gabbro  at 408 

Burned  areas, locations  of -■ - 48,49 

Burnt  Forties, banded  rocks  in 178 

granite  porphyi-y  in 25i 

location  of - ^ 

Burntside  Lake,  dikes  at 262-263 

glacial  deposits  at 428 

granite  at,  age  of - 261 

distribution  of -- 258 

foldingin - 262 

petrographic  characters  of 259-280 

relations  of .- 260-261 

granites  and   schist  at,  figures   showing  rela- 
tions of . 158,159 

metamoi-phosed  gi'eenstones  at. .      194 

view  at 392 

C. 

Cabotian  division  of  Keweenawan  rocks,  character 

and  extent  of 123 

Cacaquabic  granite,  age  of 369,406 

analysisof ' 367 

augite  of,  analysis  of 366 

character  of ---  25,365-368 

contact  of  Ogishke  conglomerate  with 305-306 

distribution  of 365 

exposui'es  of -- 369 

feldspar  of,  analysisof.. 366 

intrusion  of.  date  of .- 441 

metamorphism  of  slates  by 341 

occurrence  of 24,2.5,364 

petrogi-aphic  characters  of 365-368 

relations  of,  to  ad jacent  formations 24,368 

to  Duluth  gabbro 407 

topography  of 365 

Cacaquabic  granite-porphyry,  analysis  of 367 

Cacaquabic  Lake,  geology  of  region  near 108-109 

granite  at 364-369 

green  schist  near.. 85 

greenstone  near,  structure  of :...      136 

Lower  Huronian  slates  at 301 

Ogishke  conglomerate  at 319 

syenite  at 91 


Page. 
Cache  Bay,  Saganaga  Lake,  gi-anite  cutting  green- 
stone at 274 

greenstone  in  contact  with  contact  at 311 

Ogishke  conglomerate  at 274,309.311 

Cambrian  period,  erosion  in 445,447 

orogenic  movements  in .' 445 

Canada,  Ely  greenstone  in _ 132 

Canoe  Island,  Knife  Lake  slates  at 295 

Carbonates,  iron-bearing,  analyses  of 3S0 

Cai-p  Lake,  Agawa  formation  at 325.332 

graywacke  at 347 

slates  at 336 

Cashaway  River.    See  Kawishiwi. 
Carver,  Jonathan,  cited  on  route  from  Grand  Por- 
tage to  Eainy  Lake.. .57 

Chandler  mine,  drift  of,  view  showing 238 

mining  methods  at 2.34,239-241 

ore  of... 22,  ia3-l&5, 217-222. 241 

sections  across.... 216.217,218.219 

shaft  of,  view  showing -. 216 

Chatard,  T.    M.,  analyses  of  iron-beaidng  carbon- 
ates by 380 

Chauvenet,  W.  M.,  citation  from,  on  Duluth  gabbro.      403 
on  iron-bearing  rocks  of  gunflint  formation.      .3.'>5 

on  iron  oxide  bodies  in  Duluth  gabbro 420 

on  Logan  sills --- 399,409 

on  Pewabic  quartzite 104 

figures  cited  from --.  392,41X1,401 

geologic  work  in  Vermilion  district  done  by 3D 

Chert,  gi-een,  carbonate-bearing,  charactei-s  of .  - .  186-187 

magnetitic,  plate  showing 168 

white,  chai'acters  of - 185-186 

Chester,  A.  H.,  cited  on  iron  region  of  northern 

Minnesota 75 

exploratory  work  done  by 214 

Chester  Peak,  height  of 36 

naming  of 36 

See  also  Jasper  Peak. 

Chippewa  Indian  Reservation,  location  of 20 

Chlorite-schists,  occurrence  of.- 253 

Clark.  Thomas,  and  Anderson,  C.  L.,  report  made 
by.  on  geology  and  geological  survey  of 

Minnesota 66 

Clark,  Thomas,  and  Hanchett,  A.  H.,  citation  from, 
on  occurrence  of  hematite  at  Vermilion 

Lake 67,213 

on  route  from  Grand  Portage  to  Rainy  Lake .       57 

report  made  by,  on  geology  of  Minnesota 66-67 

Clearwater  Lake,  breccia  at 3.55 

metamorphosed  rocks  near  __ 350 

Cleavage,  slaty,  occurrence  of.. 177-178 

Clements,  J.  M.,  citation  from,  on   spherulites  in 

Michigan  rocks 1-52 

on  spilosites .- 345 

work  done  by.. 30 

Clements,  J.  M.,  and  Smyth.  H.  L.,  cited  on  crystal- 
line schist  at  Bone  Lake,  Michigan 168 

Cole,  T.  F.,  acknowledgments  to 18 

aid  by -- 3n-;il 

Cole,  G.  A.  J.,  and  Gregory,  J.  W.,  cited  on  spheru- 

litic  structure. 146 

Coleman,  A.  P.,  report  by,  containing  references  to 

geology  of  Vermilion  district 125 

Conglomerates,  occurrence  of 2-51-252 

Conglomerates   and   pseudoconglomerates,  associ- 
ation of 286 

distinctions  between 252 


INDEX. 


451 


Page. 
Contact  inetamorpliisiii,  notes  by  U.  S.  Grant  on.  125-127 

Copper  minerals,  occurrence  of _--. -  113,184 

Copper-bearing  rocks,  age  of  - 93 

cbai-acterof _ —  73-"5 

relations  of. .--  73,73-75 

CoutcMcliing,  use  of  name... 159-160 

Coutchiching  group,  equivalents  of 91 

geologic  place  of  __ 100 

Cretaceous  history  of  the  disti-ict,  sketch  of 445-446 

Cretaceous  period,  erosion  in 445-446,447 

Cross  River,  dike  at _....* 432 

Cupriferous  series.    See  Copper-bearing  rocks. 

X>. 

Denton,  F.  W.,  cited  on  mining  methods  in  Ver- 

mUion  district 234,340-241 

Diabase  dikes,  intrusion  of 443 

Dikes,  occurrence  and  character  of,  aplite 355 

basalt 34,26,306,371-373,433-424 

dolerite 300,205-306,308,400,40.5-406,433-434 

gabbro 393-394,407,408^19 

granite 33,34^35,1.56,165-166, 

199, 22.3-226, 346-274, 2&3-284, 305-341, 353-371 

gi-anite-porphyry 34-35,16.5-166 

greenstone. 155,300,223-326 

lamprophyre 308,371-373 

(luartz-porphyry -      108 

Disappointment  Lake,  greenstone  in  contact  with 

gabbroat. 161 

metamorphosed  conglomerate  at 315-316 

metamorphosed  slates  near 350 

Ogishke  conglomerate  at.. -. 318-319 

Pewabic  quartzite  at .  119 

Dolerite,  Keweenawan,  relations  of 395 

Drainage  system  of  the  district,  features  of . .  19-20, 39-46 

Drift,  extent  and  character  of 42.5-430 

Duluth  gabbro,  age  of 24,26,33 

analyses  of 405 

character  of- _  36,401-405 

dikes  in 423-424 

distribution  of  - 398-399 

exposures  of  --.- 397,399 

intrusion  of ,  date  and  conditions  of 443 

intrusive  rocks  in 42^-424 

iron  oxide  bodies  in :.. _  430-432 

metamorphismby.. 342-344,393-394,418-419 

nickel  in _. 430-421 

petrogi'aphic  characters  of 401-405 

relations  of,  to  adjacent  formations.  34,33,408-408,417 

to  Cacaquabie  granite 368.369,407 

to  Ely  greenstone... _ 4(;i6 

to  Giants  Eange  granite 357-358,407 

to  Gunflint  formation 389-390,407 

to  Keweenawan  series 407-408 

to  Knife  Lake  slates 307-308 

to  Logan  sills 410-418 

to  Lower  Huronian  sediments 407 

to  Rove  slates... 393,394,395,407 

to  Snowbank  granite ___  363-364,407 

to  Upper  Huronian  sediments 407 

stratigraphic  place  of 33 

topography  of 399-101 

Duluth  and  Iron  Eange  Railroad,  completion  of 314 

location  of ._ 53 

metamorphosed  rocks  along , 396, 340, 349 

Duluth,  Port  Arthur  and  Western  Railroad,  expos- 
ures of  Gunflint  formation  along 337, 388 


E. 


Page. 


Eagle  Lake,  Duluth  gabbroat __ 402 

Eagle  Nest  lakes,  glacial  deposits  at 428^439 

Eames,H.H.,citationfrom.ongeology  of  thedistrict       67 
on    occurrence   of   iron   ore   at  Vermilion 

Lake 313-314 

Eames,  E.  M.,  cited  on  geology  of  the  district 67 

East  Greenwood  Lake,  Cabotian  rocks  at 123 

Eby,  J.  H.,  cited  on  occurrence  of  copper  minerals 

in  hematite  ore 184 

graphite  reported  by 180 

Eby,  J.  H.,  and  Berkey,  C.  P.,  cited  on  copper  min- 
erals and  hematite  ore 113 

Elftman,  A.  H.,  citation  from,  on  Diduth  gabbro.  401,415 

on  geology  of  northeastern  Minnesota 110-111 

on  glacial  hLstory  of  Minnesota 438 

on  ore  deposits  of  Minnesota 113 

Elftman,  A.  H.,  Winchell,  K.  H.,  and  Grant,  U.  S., 
cited  on  geology  of  the  Vermilion  dis- 

ti-ict 117 

Elhpsoidal  greenstones,  deformation  of 148 

Ellipsoidal   structure   in   Ely  gre.?nstones,  occur- 

;renceof . 144-150 

plate  showing. 146 

Ely,  amygdaloidal  structure  in  rocks  near 146,165 

dikes  at  and  near 199,300,360-661,373 

Giants  Range  granite  near 354 

greenstone  exposures  near 165 

iron-ore  deposits  at 32-33 

location  of 52-53 

Lower  Huronian  sediments  at _. 399 

mining  methods  at 234,339-341 

ore  bodies  at 215 

oi-e  outcrops  at 243-344 

oresfrom,  coarseness  of 185 

panoramic  view  near 316 

population  of 20,52-53 

Soudan  formation  near .• 173 

Ely  greenstone,  age  of 33,197-198,406 

amygdaloidal  structure  in 139-141 

character  of 21,136-150,437 

colors  of 136 

conglomerate  in 138 

dikes  in. 153,356 

distribution  of 131-134 

economic  value  of 16-3-164 

ellipsoidal  structure  in _. 137,144-150 

exposures  of 134 

features  of 31,130-131 

formation  of,  date  of 4.37^38 

granite  contacts  with,  effects  of 155-157 

granite  dikes  in... 356,360-361 

jasper  areas  in 197 

jointing  in 137 

magnetite  ore  in 163 

mashing  of. 137-138 

metamorphism  of 31 , 1.55-163, 169-173, 418.419 

microscopic  chai-acters  of 150-152 

naming  of 130-131 

origin  of 154-1.55,437-4.38 

original  characters  of 136 

petrographic  characters  of 136-150 

qiiartz  veins  in c 139 

relations  of,  at  Jasper  Lake 308-309 

at  Moose  Lake 308-207 

at  Otter  Track  Lake 307-308 

to  adjacent  formations .  33, 161, 162-163, 191-199, 255- 
356, 260, 268, 274-,  381-383. 303-304, 356, 360-361, 406 


452 


INDEX. 


Page. 
Ely  grreenstone,  relations  of ,  to  Duluth  gabbru  ..  161.400 

to  Giants  Range  granite 356,359,;i60-361 

to  iron-bearing  formation - 198-199 

to  Lower  Huronian  sediments 281-283, 303-3(14 

toSaganaga  granite 268,274 

to  Soudan  formation 191-199 

to  Trout,  Bumtside.  and  Basswood   lakes 

gi-anites 260 

to  Vermilion  Lake  granite 255-256 

scMstose  forms  of,. - 153-lo4 

Soudan   formation  infolded    in,    figure   show- 
ing .. -1 - 205 

sphemlitic  and  ellipsoidal  structure  in 141-144, 

146-148, 167-168 

stratigi'apliic  position  of. 33 

structure  of 135-136 

textures  of  .- ---- 137,151-152 

topography  of 134-135 

tuffs  associated  with --  138,166 

Tolcanic  character  of 166-169 

Ely  Island,  conglomerate  at - 279,288,291 

geologic  map  of  east  end  of -- 28:i 

gi-aywacke  at - -- -  288-291 

Knife  Lake  slates  at -.- 295 

porphyry  at - 251-252,288-291 

Emerald  Lake,  brecciated  rocks  at 178 

folding  of  Soudan  formation  at 210-211 

iron-bearing  formation  at,  relations  of.. -.      210 

jasper  inter  banded  with  greenstone  at 193 

slates  at ._ --- 336 

Soudan  formation  at- --- - ---  210-211 

Ensign  Lake,  conglomerate  at -. - 319 

slates  at .- - --- 336 

Epsilon  Lake,  argillites  at  _.- 86 

Exploration  of  the  district,  history  of 56-63 

F. 

Fall  Lake,  greenstones  at  and  near 118,132 

jasper  folded  at- - 176 

jasper  interbanded  with  greenstone  at.. 192 

Soudan  formation  at .- 173 

Farm  Lake,  metamorphosed  rocks  at 340 

Fay  Lake,  Gunflint  formation  at 288 

Feldspar  of  Gacaqiiabic  granite,  analysis  of 366 

Finlay,  J.  R..  and  Smyth,  H.  L.,  citation  from,  on 

age  of  slates  of  Vermilion  district 281 

on  conglomerates  of  Vermilion  Lake 252 

on  geology  of  the  Vermilion  range 111-112 

on  iron  ores,  origin  of -      232 

on  jasper  mass  at  Soudan  Hill --      230 

on  Ogishke  conglomerates,  origin  of 2f^6 

on  schist  at  Lee  mine,  origin  of 223 

figure  cited  from - 231 

Fish  of  the  district,  kinds  of 50-52 

Flask  Lake,  cleavage  at - 303 

graywackesat .- - -      337 

greenstone,  porphyritic,  near - :il8-349 

Ogishke  conglomerate  near 317-318 

Forests  in  the  district,  area  and  character  of 20, 47-50 

Fox  Lake,  gray  wackes  and  slates  interbedded  and 

faulted  at  ..- :iiH 

Frascr  Lake.  Pewabic  quartzite  at 119 

Gr. 

Gabbro,  mc'tainon)hism  by 161-162. 1^42-344 

Gabbro  plateau  of  northeastern  Minnesota,  topo- 
graphic features  of 37-:j.s 


Page. 
GabimichigamaLake.  See  GobbemichigammaLake. 

Game  of  the  district,  kinds  of  ..- 50-52 

Garden  Lake,  Soudan  formation  at .- 173 

Geikie,    Archibald,  citation   from,    on   ellipsoidal 
structure   in   volcanic   rocks  of  Great 

Britain- -  144-145.150 

Geographic  location  of  the  district  - 32-33 

Geologic  history  of  the  district,  sketch  of 437-447 

Geologic  structure,  relations  of.  to  topography...  431-436 

Giants  Range,  Animikie  rocks  in 93 

area  north  and  northwest  of,  topography  of 36-37 

course  and  general  features  of.-- 35-36 

glacial  deposits  near 426-428,429 

origin  of.. - 444 

relations  of  topography  and  geology  at .  - 431^36 

iSee  o/.so  Mesabi  range. 

Giants  Range  granite,  age  of 358,406 

aplite  dikes  in 355 

conglomerate  in  contact  with 305-306 

dikes  in 355 

distribution  of _ 353-3.54 

exposures  of ^4 

folding  in 358-359 

intrusion  of,  date  of 441 

in  the  Ely  greenstones 165-166 

Knife  Lake  slates  in  contact  with 340-342 

gi'anite,  metamorphic  action  of 24,359 

petrographic  characters  of 354-356 

relation  of,  to  adjacent  formations 356-358 

to  Duluth  gabbro _ - 407 

to  Ely  greenstone.- 356,859,360-361 

to  Keweenawan  gabbro  - 357-358 

to  Lower  Huronian  sediments 356-357 

to  sedimentary  rocks 283 

to  Soudan  formation 356,359 

schists  in  contact  with 171 

topography  of - 354 

Glacial  drift,  extent  and  character  of 425-430 

Glacial  lakes,  occurrence  of 127.429-430 

Glaciationiu  the  district 27,39 

Gobbemichigamma  Lake.  Animikie  series  at 88 

conglomerate-greenstone  contact  at 304 

depth  of .- -- --.■ 46 

dike  cutting  gabbro  at  .- 423 

Duluth  gabbro  at -.- ."" 39S 

folding  at --.  306-307 

gabbro-slate  contact  at 351-352 

greenstones  at IS.  133. 136 

greenstone -gabbro  contact  at -.-      161 

Gunflint  rocks  near 383 

iron-bearing  formation  near 383 

Logan  sills  at j 398 

Lower  Huronian  sediments  at . .  - 298, 301 

metamorphosed  slates  at 343 

Pewabic  quartzitesat j.19 

L"'i)ijer  and  Lower  Huronian  at,  relations  of 306 

slatesat 343,351-352 

topography  and  geology  at 43:^ 

unconformity  at 307 

Gold,  reported  occurrence  of 213 

Gold  mining,  early  attempts  at,  in  Vermilion  dis- 
trict   - 213 

(Told-bearing  rocks.  Vermilion  Lake,  occui'rence  of.       69 

Graff.  C.  F..  acknowledgments  to 31 

Grand  Portage,  rocks  at - 122 

:   Granite,  age  of 90.24(^24*' 

chanicter  of ^2. 107-ia^.  124. 24(>-250. 258-205. 266  -an* 

dikes  of 255-256 


INDEX. 


453 


Page. 

Gtranite,  distribution  of  - --- 90 

relations  of 254-2.56, 2.59-261, 264-265, 268-27.3, 282-283 

Kawishiwi  Eivev  area,  distribution  of _  263-264 

exposures  of .- 264 

petrograpliic  cliaracters of.. 264 

relations  of-. 264-265 

Knife  Lake  area,  occurrence  of  J .-.        24 

Moose  Lake  area,  distribiition  of 26.3-264 

exposures  of 264 

petrographic  characters  of 264 

relations  of. 264-265 

Saganaga  Lake  area,  distribution  of 266 

exposiires  of _ 266 

metamorphic  effects  of. _ 273 

petrograpbic  characters  of 266-267 

relations  of 268-273 

topography  of 266 

Trout,  Burntside,  and  Basswood  lakes  areas,  age 

of 261 

distribution  of-. 258 

exposures  of  -. 2.58,262-263 

folding  in _ 262 

petrographic  characters  of 259-260 

relationsof 260-261 

topography  of 2.59 

Vermilion  Lake  area,  distribution  of ._      247 

exposures  of _ _ 247-248 

folding  in 250-251 

petrographic  characters  of 248-2.50 

relations  of,  to  adjacent  formations 254-2.57. 

282-283 

topogi'aphy  of 248 

sti'ucture  and  metamoi-phism  of __.  251-254 

Grant,  U.  S.,  citation  from,  on  Cacaquabic  granite.      364 

365, 366,  .367 
on  contact  metamorphism  in  jVIinnesota . .  12.5-127 
on  cordierite  at  Gobbemichigamma  Lake. . .      346 

on  Duluth  gabbro 401,402 

on  dikes  in  Duluth  gabbro 422 

on  gabbro  plateau  of  northeastern  Minne- 
sota.  37-38 

on  glacial  lakes .■ 430 

on  granitic  areas  in  Minnesota 107-109 

on  Gull  Lake  rocks 312 

on  Gunflint  formation. 390 

on  Gunflint  Lake  area. 38,9.3,109-110 

on  iron  ores  of  Gimflint  beds 385 

on  jasper  in  slates  at  Pickle  Lake 325 

on  Kawishiwi  River 41 

on  Logan  sills ,399 

on  Mesabi  iron  range,  geology  of 114-116 

on  Logan  sills  and  Duluth  gabbro,  relations 

of..., 410-416 

on  metamorphism 12.5-1.26 

on  muscovado 343 

on  Minnesota  geology 118-125 

ov  Ogishke  conglomerate 324 

onSpganagaLake  granite 265,  267,  269 

Grant,  U.  S.,  Winchell,  N.  H.,  and  Blftman,  A.  H., 
cited  on  geology  of  the  Vermilion  dis- 
trict       117 

Graphitic  rocks,  ocurrence  of 179-1  SO 

Great  gabbro,  stratigraphic  place  of IM-ini 

See  also  Duluth  gabbro. 
Green,  R.  B.,  table  prepared  by,  showing  coarseness 

of  iron  ores \.      185 

Greenalite,  occurrence  of I9fi 


Page. 

Greenstones,  amy gdaloidal 168-169 

belts  of ■ 19,5-196 

ellipsoidal 144-150 

deformation  of 148 

inclusions  of,  in  iron-bearing  formation 197 

relations  of,  to  jasper 200-210 

schistose,  occurrence,  and  character  of 1.53-154 

spherulitic 141-144 

Gregory,  H.  R.,  cited  on  ellipsoidal  structure  in 

Maine  andesites 144 

Gregory,  .J.  "W.,  and  Cole,  G.  A.  J.,  cited  on  spheru- 
litic striicture  146 

Grtlnorite,  origin  of 187 

Gull  Lake,  granite  and  greenstone  at 274 

greenstone  and  conglomerate  in  contact  at 3.24 

Gunflint  formation,  age  of .33,406 

character  of 25,377-387 

deposition  of.  time  and  conditions  of. 443 

distribution  of , 375 

equivalence  of  Biwabik  formation  to 25,377-379 

exposures  of 376 

granules  in,  photonaicrograph  showing 382 

iron  ores  of,  analyses  of .. 385 

iron-bearing  rocks  of... 377-387 

metamorphism  of 419,444^445 

petrographic  characters  of 377-.387 

relations  of,  to  adjacent  formations 387-.390 

to  basic  dikes  cutting. ,390 

to  Duluth  gabbro 407 

to  Keweenawan  gabbro 389-390 

to  Knife  Lake  slates 

to  Lower  Huronian  series 

to  Ogishke  conglomerate. 388-389 

stratigraphic  equivalent  of 25 

stratigraphic  position  of .33 

structure  of 73,376-.377 

thickness  of . 390 

topography  of 376 

Gunflint  Lake,  Animikie  rocks  at 74,84,86,93 

argillitesat 86 

banded  rocks  at 149 

geology  of  region  near 65,73,109  ' 

greenstones  near ng 

Gunflint  formation  at 387 

hematite  at 72 

Huronian  rocks  at 71 

iron-ore  deposits  at 213 

jasper  at 73 

Keewatin  slates  at 93 

Logan  sills  at 399 

sections  at ...^ 392,400 

syenite  at 91 

Vermilion  schist  at 93 

topography  near ._,  38.39 


H. 


Hall,  C.  W.,  citation  from,  on  Arehean  rocks  of  Min- 
nesota   ..■ 116-117 

on  geology  of  the  district 70-71 

on  granites  of  the  Northwestern  States. 90 

Hanchett,  A.  H.,  and  Clark,  Thomas,  citation  from, 
on  occuiTence  of  hematite  near  Vermil- 
ion Lake 213 

on  route  from  Grand  Portage  to  Rainy  Lake ...       57 

report  made  by,  on  geology  of  Minnesota. 66-67 

Harvey,  H.  E.,  iron.ore  outcrops  discovered  by 215 


454 


INDEX. 


i'age. 
Hematite,  features  of 182 

Hematitic  specular  iron  ore,  occurrence  of 67 

Henry,  Alexander,  citation  from,  on  ti*avels  in  the 

Northwest _ 63 

BDighest  point  in  district,  location  of 34,  :i5-36 

Hitchcock,  Edward,  cited  on  schist  conglomerates 

of  Vermont -      170 

Houghton,  Douglas,  geologic  work  in  Lake  Superior 

region  done  by 64 

Himt,  T.  Sterry,  Animikie  series  named  by 374 

Huronian  group,  components  of 76,80-81,88,89-90 

Huronian  quartzite ,  relations  of 93 


Iddings,  J.  P.,  citation  from,  on  crystallizations  in 

acid  lavas 143 

on  lavas  of  Yellowstone  Park 143 

Igneous  intriisions,  periods  of 4iiT-147 

Indian  reservation  in  the  district,  location  of 20, 54 

Indians  in  the  district,  number  and  character  of. . .  20, 54 

Ingall,E.D., citation  from, on  Animikie  series 396 

on  Logan  sills 408 

Intrusive  rocks  in  Duluth  gabbro,  occurrence  and 

character  of _ __  422-424 

Iron-bearing  carbonates,  analyses  of 380 

Iron-bearing  formation,  age  of ' 195-196, 200 

banding  of 188-190 

belts  of 195-196 

characters  of 22 

clastic  rocks  associated  with 212 

deposits  at  bottom  of 223-224 

deposits  within 234r-227 

greenstones  associated  with 196 

inclusions  of,  in  greenstones 197 

interbedded  volcanic  greenstones  with 196 

macroscopic  characters  of 181-183 

mici'oscopic  characters  of ^ 185-188 

origin  of 188-191 

relations  of,  to  adjacent  formations. . .  197-198, 207-208 
unconformity  between  Ogishke  conglomerate 

and -.      307 

Iron  Lake,  Duluth  gabbro  at 403 

Iron  Mountain  Lake.    See  Ensign  Lake. 

Iron-ore  deposits,  age  of 232-234 

character  of 101-103, 113, 137-128, 183-185 

distribution  of 78-79 

geologic  horizons  and  relations  of. . .  96-97, 127-128, 315 

historical  sketch  of  exploration  of 2i:3-215 

localities  of 67,96-97 

methods  of  mining 234-341 

origin  of -  78-79,95,100,101-103,227-234 

prospecting  for,  mode  of 24:j-246 

Iron  ores,  analyses  of 385 

eharacterof 101-103,113,183-185 

coarseness  of _      185 

copper  minerals  in 184 

iron  content  of 184 

methods  of  mining 334-241 

phosphorus  content  of... 184 

production  of 241-243 

shipments  of 2)5,241-243 

silica  content  of 184 

Iron  oxide  bodies  in  Duluth  gabbro,  analyses  of 420 

eharacterof 420-432 

Irving,  R.  D.,  citation  from,  on  Animikie  series...  74,396 

on  Archean  formations  of  Minnesota 77-78 

on  Cain))r  jui  iind  iir<'-(.'anil)riiLn  formations.  89-911 


Page. 
Irving,    R.   D..  citation    from,  on    copper-bearing 

rocks _ 73-75 

on  Huronian  group _ 80-81 

on  iron-bearing  schists 71 

on  ii'on  ores  of  Lake  Superior  region,  origin 

of.. 78-79 

on  Logan  sills 408 

on  relations  of  Duluth  gabbro  to  Kewee- 

nawan  series 407 

geologic  work  in  Vermilion  district  done  by 30 

Irving,  R.  D.,  and  Van  Hise,  C.  R.,  citation  from,  on 

Animikie  slates 396 

on  Gunflint  beds. 379 

on  iron  ores,  origin  of 228 

on  mica-schist  at  Enghsh  Lake 350 

cited  on  Penokee  iron-bearing  series 97, 390 


Jasper,  brecciation  of ._  194-195 

color  of 182 

folded  and  contorted,  plates  showing 176,178 

relations  of,  to  greenstones 200-210 

unhanded,  occurrence  of _..      197 

Jasper  Lake,  spherulitic  greenstones  at  and  near    143, 

148,168 

iron-bearing  formation  at,  relations  of , 208-209 

jasper  inter  banded  with  greenstone  at 192-193 

Jasper  Peak,  height  of. ._ 1___ 36 

jasper  at _ 311 

Soudan  formation  at 173,175 

Kawishiwi  River,  course  and  character  of 40-41 

conglomerates,  metamorphosed,  near 316-317 

dikes  along 357 

gabbro  near 93,407 

granite  along  and  near 363-364, 355,  a57,;)59-360, 407 

greenstones  near 118 

ellipsoidal,  near 169 

metamorphosed  rocks  along  . . ,  316-317, 340-341, 359-360 

rocks  of  various  kinds  near 108 

Kawishiwin,  name  proposed 91 

Kawisbiwin  agglomerate,  character  of 104 

Kawishiwin  greenstone,  age  and  geologic  relations 

of 91-118 

Kawishiwin  rocks,  components  of  (note). 131 

6'efo/so  Ely  conglomerate  ojjrfSoudanformation. 

Keewatin  series,  age  of 11)6-107.114 

Kewatin  series,  eharacterof.. 96,106-107,114-115 

deposition  of,  conditions  of . 91 

ironores  of,  mode  of  origin  of 95 

origin  of 88,93 

relations  of 87,  K)5, 114-115 

stratigi-aphic  place  of 91-92, 100,  l(Hi-107 

Kekekabic,  Kekekebic.  Kekequabic.    Set-  Cacaqua- 

bic. 
Keweenawau  dolerite,  relations  of  Rove  slates  to  . .      395 
Keweenawan  gabbro,  metamorphism  of  slates bj'.  342-344 

relations  of,  to  Cacaquabic  granite 368,369 

to  Giants  Range  granite 357-3.58 

to  Gunflint  formation 389-;i90 

to  Knife  Lake  slates 307-^08 

to  Ogishke  conglomerate 307-^308 

to  Rove  slates 395 

to  Snowbank  granite 363-364 

See  o/.vo  Duluth  gabbro. 


INDEX. 


455 


Page. 

Keweenawan  series,  age  of -. -      106 

Cabotian  division  of --- --  123-124 

character  of — - -  26-27,106 

components  of 100,122-124 

dikes  in. — -  422-434 

distribution  of 

Duluth  gabbro  of.. 

equivalents  of US 

exposures  of 297,299 

formations  composing - -       ^ 

gabbroof- - -- 124,397-422 

igneous  rocks  of ---      123 

intrusive  rocks  in 422-424 

iron  oxide  bodies  in ._ 420-422 

Logan  sills  of.. 397-422 

Manitou  division  of -- --      124 

metamoi-phism  by ..-  418-419,424 

origin  of -- 443-444 

petrographic  characters  of 401^06,423 

Potsdam  rocks  incliided  in... 132-123 

Puckwunge  conglomerate  of 122 

relations  of,  to  adjacent  form,ations 73-75,408-416 

to  Logan  sills _ 410 

stratigraphic  place  of -. 89,99 

topography 399-401 

Kloos,  J.  H.,  cited  on  geology  of  the  district 69 

Kloos,  J.  H.,  and  Streng,  A.,  cited  on  geology  of  the 

district -        70 

Knife  Lake,  Agawa  formation  at 325 

Ely  greenstone  at 133,136 

jasper  interbanded  with  greenstone  at 193 

Lower  Huronian  rocks  at  and  near 23,297-303 

topography  and  geologry  at,  relations  of 433,434 

slates  at  and  near 300,301 

Knife  Lake  slates,  age  of 33,73,384,308,406 

characters  of 393-296, 336-a39, 344-346 

dikes  in 308,341 

dips  and  strikes  of 280 

exposures  of _. 295-296,347-352 

features  of. __ _ 34 

folding  and  faulting  in 301 

granite  contacts  with 340-342 

metamorphism 301,340-344,349-351,445 

naming  of 335 

origin  of .  _ 44X 

petrographic  characters  of 293-296, 336-339, 344-346 

stratigraphic  position  of 33 

pyritein _      394 

relations  of,  to  Agawa  formation 303 

to  Gunflint  formation 388-389 

to  Keweenawan  rocks 307-308 

to  Ogishke  conglomerate... 281,303 

to  Snowbank  granite _-. 363 

strikes  and  dips  of. 280 

thickness  of 295,346-547 


r.. 


Lac  Bois  Blanc.    See  Basswood  Lake. 

Lake  Gobbemichigamma.    See  Gobbemichiganima 
Lake. 

Lake  Superior  Basin,  origin  of 444 

Lake  Superior  region,  pre-Cambrian  history  of . .  105-106 

Lake  Vermillion.     See  Vermilion  Lake. 

Lakes  of  the  district,  character  of.. 41-43 

depths  of 45-46 

origin  of. _ 43-46 


Page. 

Lakes,  glacial,  occurrence  of 429-430 

Lauren tian  rocks,  characters  and  components  of  ..      76, 

89, 90-91, 107, 113-114 

Lawson.  A.  C,  citation  from,  on  Coutchiching 158-160 

on  ellipsoidal  structure  of  rocks  of  Lake  of 

the  "Woods  region 149 

on  granite  of  Saganaga  I^ake 265, 268-269, 345 

on  greenstones  of  Rainy  Lake  region 156 

on  Lake  Superior  stratigi'aphy 99-100 

on  Logan  sills 398,408,409-410 

Lee  Hill,  breccia  at.. 178,330 

conglomerate  at 287 

graphitic  slates  near 179 

greenstones  and  jaspers  at 303-303 

Soudan  formation  at 173, 175, 201 

topography  and  geology  at,  relations  of 431 

Leith,  C.  K.,  acknowledgments  to 56 

citation  from,  on  Biwabik  formation 378-379,390 

on  dikes  in  Embarrass  granite 357 

ongreenalite 127,190 

on  griinerite 187 

on  Gunflint  and  Lower  Huronian  rocks,  re- 
lations of 389 

on  metamorphism  of  slates  by  gabbro  in 

Mesabi  range. 394 

on  Ogishke  conglomerate  and  iron-bearing 

formation,  relations  of 307 

on  secondary  minerals  in  schists 161 

on  spherulitic  structure  in  rocks  of  Mesabi 

district 143 

work  done  by 17,30 

Literature  of  the  district,  resume  of 56-128 

Little  Saganaga  Lake,  gabbro  at.. 402-403 

Logan,  W.  E  ,  cited  on  Logan  sills 398 

Logan  sills,  age  of , 33 

characters  of 26,405-406,411-416 

distribution  of 398-399 

exposures  of 397-398,399 

intrusion  of 443 

metamorphism  by ...  418-419 

peti'ographic  characters  of 26,405-406,411-416 

relations  of.  to  adjacent  formations 133, 408-418 

section  showing  intercalation  oJ,  in  Rove  slates.      400 

stratigraphic  place  of 33 

topography  of 399-401 

Long  Lake,  amygdaloidal  rocks  at 146 

breccia  at 363 

dikes  at 373 

gold-bearing  quartz  at 164 

granitic  intrusions  at 363 

greenstone  exposures  near. 165 

spherulitic  greenstones  at _. 143 

Loon  Lake,  gabbro-slate  contact  at 394,395 

Lower  Huronian  intrusive  rocks,  acid  dikes  of. . .  369-371 

character  of 23-34 

distribution  of....  24^25, a>3-354,;361, 365, 369-370, 371-373 

exposures  of 354, 359-361, 364, 365, 369, 373 

relations  of 370-371 

topography  of .^...  354,361,365 

Lower  Huronian  sediments,  age  of 23,308 

areas  of 23,277-296 

characters  of 24-35, 

384^295, 309-313, 327-328,  a36-339, 344-346 

dikes  cutting 306,356-357 

divisionscf 23-24,33,275-276 

exposures  of 278,298-399 

formations  composing 23-24, 33, 275-276 


456 


INDEX. 


Lower  Huroniau  sediments,  granite  dikes  in 3oQ-'So7 

Knife  Lake  area  of - ---  297-849 

metanioi"phism  of -- -- --      419 

relations  of,  to  adjacent  formations . . .  281-284, 303-308 

to  dikes -  283-284 

to  Duhitli  gabbro - -.-      407 

to  Giants  Range  granite  __- 254,2S3,35&-357 

to  granite  of  Vermilion  Lake 254-256 

to  Gunflint  formation _ 388-389 

toSaganaga  granite - 268-273 

to  Snowbank  granite - 363 

to  Upper  Hm-onian._ .- 97-98,306-307 

petrograpbic  cbaractei*s  of  -. - 284^295, 

309-313, 327-328, 336-339, 344^-346 

stratigrapbic  place  of  ...,. 308 

structure  of --- 278-2SL 299-3 13 

Vermilion  Lake,  area  of 277-296 

Lower  Kewatin  rocks,  occurrence  and  character 

of ---  118-119 

Lowest  point  in  district,  location  of 34 

Mackenzie,  Alexander,  explorations  made  by 57-63 

Magnetitic  chert,  plate  showing -      168 

Mallmann,  John,  exploratory  work  by. 214 

Manitou  rocks,  character,  thickness,  and  extent  of.      124 

Marquette  district,  iron-ore  deposits  in 22 

Marqiiette  series,  character  and  thickness  of 88-89 

eciuivalents  of --- - 92 

Marquettian,  name  proposed - 88 

Maurer,E.  R.,  acknowledgments  to 31 

Mayhew  Lake,  Duluth  gabbro  at 403 

Merriam,  W.  N.,  citation   from,  on  iron-bearing 

rocks  of  Gunflint  formation •  385 

on  Logan  sills 399 

geologic  work  in  Vermilion  district  done  by 17, 

30.31.33 
sketches  of  folds  in  Soudan  formation  made  by.      176 

Mesabi  range,  course  and  general  features  of 35-36 

geology  of- -  114-116 

origin  of  name  of _ 35 

metamoi'phic  contact  of  gabbro  and  slate  in 1^4 

See  also  Giants  range. 

Mesabi  series,  equivalents  of _ 92 

occui'rence  of 121 

Metabasalt,  plate  showing 168 

Mica-schist  derived  from  greenstones,  composition 

of - 157 

Mining,  methods  of - ^-- 234,241 

Minnesota,  copper  minerals  of _      113 

crystalline  rocks  of .- 106-107 

ore  deposits  of 113 

Minnesota  Geological  Survey,  reports  of,  citation 
from,  on  altitude  of  highest  point  in  dis- 
trict         34 

citation  from,  on  Mesabi  and  Giants  ranges.       35 

on  island  in  Kawishiwi  River 41 

on  Sttintz  conglomerate 278 

on  Ogishke  conglomerate 304 

on  Agawa  formation 325 

on  Cacaqiiabic  granite 364 

on  titaniferous  magnetite  at  Iron  Lake 403 

on  Logan  sills 409 

on  augito-granite  cut  by  hornblende  granite .      :^2 
See  also  Winchell.  Grant,  and  other  Minnesota 
geologists. 


Page. 

Minnesota  Iron  Company,  view  showing  filling  sys- 
tem of 236 

Minnesota  mine,  view  of  main-level  timbering  at . .      236 

Mishiwishiwi.    See  Kawishiwi. 

Montalban  group  of  Vermilion  district,  rocks  com- 
posing   76 

Moose  Lake,  Agawa  formation  near 325, 

326,329,330.333-335 

amygdaloidal  rocks  at : 146 

breccias  near 137, 305 

conglomerate  at _. 206,304,317 

conglomerate  and  greenstones  at,  relations  of . .     206, 

303,304 

dikes  at 261 

Ely  greenstone  near,  relations  of 132, 206 

glacial  deposits  near 428 

granite  at  and  near. 2ol,264r-265 

gi'eenstone  at 132,1^5-136,206 

greenstone  conglomerate  at 313 

greenstones,  ellipsoidal,  at. 148-149 

greenstones,  porphyritic,  near __ 348-349 

iron-bearing  formation  near,  relations  of 205-207 

jasper  infolded  in  greenstone  at 192 

Lower  Huronian  rocks  at _  297,298 

Ogishke  conglomerate  at  and  near 206, 303^  304, 317 

reibungs-breccias  at 305 

rocks  at,  figure  showing 206 

slates  at 302.336,337 

slate  fragments  in  conglomerate  at 304 

Soudan  formation  near 172 

structure  at 135 

topography  and  geology  at 433 

Mud  Creek,  granite  dikes  near _ 256 

Mud  Creek  Bay,  chert  veins  in  acid  porphyry  at . ..      191 

conglomerate  at 288 

granite  dike  in  greenstone  at 255 

granite-porphyry  dike  at 257 

granitic  intrusions  in  schist  at _ 262 

intrusive  rocks  in  iron-bearing  formation  at 199 

Muldrow,  Robert,  work  done  by.- _       17 

Muscovado,  occurrence  of,  at  Gobbemichigamma       • 
Lake 88,343 

Muskegs,  occurrence  and  character  of 33 

nsr. 

Ne wf oiind  Lake,  Agawa  formation  at .■- 335 

greenstone  and  granite  at 172 

glacial  deposits  near 428 

graywackes  at. 337 

metamorphism  at 172 

Nickel,  occurrence  of,  in  Duluth  gabbro.. ._ 421 

Nipigon  series,  age  and  character  of 106 

stratigraphic  place  of 99 

See  also  Keweenawan. 

North  Twin  Lake,  jasper  near _ 189 

spherulitic  greenstones  near _ 142,168 

Norwood,  J.  G.,  cited  on  iron  ore  in  the  district 29 

geologic  work  in  district  done  by 65-66 

publication  of  occurrence  of  iron  ore  by 22,213 

Norwood  Lake,  site  of 430 

O. 

Oak  Lake,  arkose  at 270 

Ogishke  conglomerate,  age'of 33, 1(15, 284, 308, 406 

anticlinal  position  of 279 

character  of 23-24,81,284-285,309-313 


INDEX. 


-457 


Page. 

Oglshke  conglomerate,  dikes,  basic,  in 308 

dikes,  granite,  in 305-:*6 

divisions  of 84-85 

exposures  of - 287-293,317-324 

origin  of - --- 2&5-286,44n 

metamorpliisni  of 313-317 

petrographic  charaotei-s  of  284-285 ,  309-313 

relations  of,  to  Agawa  formation 303 

to  Cacaquabic  granite 305-306 

to  Ely  greenstones 206-207,303-304 

toDuluth  grabbro 307-308 

to  Gunflint  formation -.  388-389 

to  Keweenawan  rocks 307-308 

to  Knife  Lake  slates- ■-      281 

to  Saganaga  granite 305 

to  Snowbank  granite .--      353 

to  Soudan  formation ._ 205-207 

to  Stuntz  conglomerate 88 

slate  fragments  in  ._ 304 

stratigraphic  place  of 33, 81, 87, 88, 105, 284, 308, 406 

thickness  of  -  - 286-287,81? 

Ogjshke  Munoie  Lake,  Agawa  formation  at . . .  323, 327, 328 

conglomerate  at  and  near ■  72, 

81, 84-85, 102-103, 276, 284-293, 310, 320-324 

geology  of  region  near 73 

greenstones  and  conglomerates  at,  relations  of.     304, 

332-323 

greenstone  near,  structure  of 136 

Lower  Hvironian  sediments  at... 298 

Lower  Kewatin  rocks  at --.      119 

schistosity  of  rocks  at 302 

Ontarian  system,  geologic  place  of 100 

Orogenic  moTements.  historical  sketch  of 437-447 

Otter  Track  Lake,  Ely  greenstone  at 193,207-208 

iron- bearing  formation  at.  relations  of 207-208 

jasper  at - - --  193,310 

Soudan  formation  at,  relations  of 207-208 

topography  and  geology  at,  relations  of 434 

Owen,  David  Dale,  geologic  work  in  northwest  done 

by -, 65-66 

iron-ore  deposits  first  mentioned  in  report  of. . .      313 

Paul  Lake,  Lower  Huronian  sediments  at 300 

spotted  rocks  at _.- 345-346 

Paulson  Lake,  Duluth  gabbro  at 398 

Paulson's  mine,  Gunflint  rocks  at 375 

Pengilly,  cited  on  occurrence  of  native  copper 184 

Penokee  series,  character  and  relations  of 97 

equivalents  of 92 

Pewabic  quartzite,  chai-acters  of 103,110 

occurrence  of 119 

relations  of__ 110 

stratigraphic  place  of.  _•- 111,U8 

thickness  o. _. 102 

See  aJso  Biwabic. 

Physiography,  features  of 34-46 

Pickle  Lake,  Agawa  formation  at 325, 333 

Pike  Bay,  conglomerate  on  islands  in 287 

Pike  River,  rocks  exposed  near. . -.-  65,68,296 

Pine  Island,  Vermilion  Lake,  glacial  deposits  at 428 

Knife  Lake  slates  at — 395 

Pine  Lake,  greenstones  near 132 

Pioneer  mine,  ore  body  at ----  317-318 

ores  in,  coarseness  of 185 

shaft  of,  view  showing 216 


Page. 

Plankton  of  lakes  in  the  district,  collections  of 46 

Pokegama  quartzite,  character  of 121 

Potsdam  rocks,  equivalents  of,  in  Vermilion  dis- 
trict...  76,92 

Pttckwunge  conglomerate,  stratigraphic  place  of  . .      122 


Ti. 


Rainy  Lake,  Lower  Kewatin  rocks  at 119 

Range  (Vermilion  iron),  explanation  of  use  of  term.       34 

Range  1  west,  township  63  north ; 123 

R.  2W.T.  63N 123 

R.  2W.,  T.  64N... 133 

R.  4W.,  T.  63N : 123 

E.  4  W.,  T.  65  N.,  section  21 37.5,377 

section  22 375,377 

section 23 375,387,390 

section  24 387 

section26 375,390,391 

section  27 119, 133, 161, 375, 377, 388 

section  28 34,35,161,398 

section  29 161 

section  30 161,298,328 

section  34 377 

R.  5'W.,  T.  64N 352 

section  6... 3.51,423,4.33 

R.  5W.,  T.  &5N 253 

sections 269 

section  7 .-      271 

sections  22,23 274 

section  25 ^ 388 

sections  26, 27 389 

sections  29, 30.... 398 

sections  31, 32.-. 433 

section33 390 

section  34 119,377,435 

section  35 435 

R.  5  W.,  T.  66  N... 428 

section  30 269,434 

R.  6W,,T.  60  N -- 123 

R.  6W.,  T.  64N.... 433 

section  1.. 161,351,433 

section  2 161 

R.  6W.,  T.  65K 432 

sections  3, 10 434 

sections  11, 18, 19, 25 133 

section  26 133,321,323 

section  27 133,302,321,323 

section28 319 

section  29 325,332 

section  30 325,333 

section  31 133 

section  35 301 

R.  6W.,  T.  66  N 438 

sections  24,  25,  26,  34 434 

section  35 301,434 

R.  7W.,  T.60N .'. 123 

R.  7  W.,T.  64N 433 

section  1 365,369 

section  3 365 

R.  7W.,  T.  65N-. .—      432 

section  21 133 

sections  22,  23 433 

sections  25,36 133 

R.  8W.,  T.  64N.. 361 

section  11. 349 

sections  36,  35 161 


458 


INDEX. 


Page. 

R.  9  W..  T.  liSN 133,361,427,43-2 

section! 172.192.2C6 

section  5 263 

section  G 142,168 

section  10 -.-      341 

section  1.5 435 

section  16 316 

sectionl7 ia3, 316,  .341, 34»,  373 

section  19 353,357,458 

section  20 169,2as,316 

R.  9  W.,  T.  64  N 133,361,428.4.32.433 

section  10 __. 132 

section  16 132,172,261,263,337 

section  17 263 

section  19 407 

section  32 146 

section  .33 146,207 

section  33 362 

R.IOW.,  T.  58N 123 

R.  low..  T.  62N.,  sections 398 

section  30 119 

R.  Ill  W.,  T.  63N. 138 

section  1 142 

sections  3-5,  8,  9 427-12S 

section  10 142, 167, 189, 199, 360, 427-428 

section  11 142,167 

section  12. ...' 142,168 

sections  14, 15.. 142.360 

sections  16, 17 360 

section  20 166,360 

section  21 360 

sectioif24 .3.53,355 

section25 353 

section  26 398,435 

section  29 298,359 

sections!) 340.360 

section  31 360 

section  34 357 

section  a5 398 

R.  low.,  T.  64N 428 

.    sectionl9 168,263 

section31 132 

R.  11  W..T.  .55]Sr 123 

R.ll  W.,  T.  62N.. 354 

R.  UW.,  T.  6:3N.. 173 

sections 148 

section  7 : 148,192 

section  26 340 

section  30. 166,173,174,175.427 

section  31 427,436 

section  34 357 

section  36 360 

R.  12  W.,  T.  62  N.. .55.354 

section  1 360,436 

section2 37,380 

sections 166,199,200,359,360,436 

section4 199,200,359 

section  7 359,360 

sections 360 

section  9 436 

section  11 354,436 

section  17 •. 360, 4«) 

section  18 199,360 

section  19. 167,359,360 

section  .30 37 

R.  13  W.,  T.  63  N.,  section  9 146,147 

section  10 142,147 


Page. 

R.  12  W.,  T.  &3N.,  section  11  : 142 

section  13 192 

sections  14-16 142 

section  21 142,263 

^ection22 142 

section  25 173,174,175,212,427 

section26 427 

section  30 164 

sections  34-36.. 427 

R.  12  W.,  T.  64  N.,  section  36 435 

R.  12  W..  T.  66  N.,  section  7 37 

R.  13  W.,  T.  61  X.,  section  6 427 

R.  13W.,T.  62N 428 

sections 166 

section! 132.297.332 

section  7 359,360 

section  14 427 

section  15 194,427 

section  17 166,188 

section  18 171 

sections  22,23 427 

section  24 1 167,359,360 

sections26,27 436 

section  28 171,359,360,436 

section  29 171,4:36 

section  31 171,353 

section  32 171,194,353 

T.  63  N.,  sections  20,30... 263 

section  32 435 

T.  66  N.,  sections  11, 12 37 

T.  60N 429 

T.  61N 428,429 

sectionsl,2. 427 

section  4 175 

section  10 1.56.427 

sections  11, 12, 14,  1.5,  16.18,19,21,22, 

29,30 247 

T.  62N 166,428,429 

section2 277 

section  3  _ 435 

sections 204 

section6 174,204,255 

section? 174,175,199,204.257 

sections 174 

section9 2-56 

section  10 212,256 

section  15 131,277 

sections  16-18 131 

section  20 256 

section  21 1.31,197,256 

section  27 256 

section35 168 

R.  15W.,  T.  60N 429 

R.  15W..  T.  61N 131,428,429 

section  1 197,20:3 

section  2 197 

sections 174,197,203 

section  4 174 

section  6 287,292 

sections  21-28 427 

R.  15  W..  T.02N'.. 173.428 

section  1 1:31,174,199,204,255,256 

section  2 1:31,43:3 

sections  3, 4 -. 1:31 

section  7 131.256 

sections8,9, 12 131 

section  14 287 


R.  13  W, 

R.  13  W. 
R.  14  W. 
B.  14W. 


R.  14  W. 


INDEX. 


459 


Page. 

R.  15  W.,T.63N., section  20 202,295 

section  S2 3T3 

section  27 200 

section  28 -...      301 

section  35 36,131 

section  36 131 

E.  15W.,  T.  63N-. - —      435 

sections  34, 35 - 433 

R.  IIJ  W.,  T.  60N 429 

R.  16W.,  T.  61N 277,429 

R.  16  "W".,  T.  G2  N .-..: 277 

R.  IGW.,  T.  63N.. 277 

Ransome,  F.  L.,  cited  on  spherulitic  rocks  of  Cali- 
fornia   143 

Reibungs-breccia,  occurrence  of 137,178 

Relief,  features  of 34-35 

Rivers  of  the  district 40-41 

Roa  ds  in  tlie  district,  character  of 53-54 

Robinsons  Lake,  rock  banding  near 195 

Bohn,  Oscar,  aid  by 31 

Rose  Lake,  view  at 392 

Round  Lake,  metamorphosed  slates  near 350 

Routes  of  travel  in  the  district,  character  of 56-63 

Rove  slate,  age  of 33,396,406 

deposition  of,  time  of 442 

distribution  of 391 

exposures  of 391 

gabbro  contact  with.. 394 

metamoi-phism  of 393-394,419 

naming  of 390 

occurrence  of... 25 

petrographic  characters  of 392-393 

relation  of,  to  Duluth  gabbro 407 

to  Keweenawan  dolerite  and  gabbro. 395 

section  through,  showing  intercalated  Logan 

sills.. 400 

stratigi*aphic  position  of 26,33 

structure  of 392 

thickness  of 396 

topography  of _. 391-392 

Royal  Commission  on  Miuei*al  Resources  of  Ontario, 

report  of,  cited _       94 

S. 

Saganaga  Lake,  arkose  at 270 

conglomerate  at 85,274,309 

Ely  greenstone  and  Ogishke  conglomerate  at, 

contact  of sll 

geology  of  region  near 65 

granitoid  and  gneisso'd  rocks  at 87 

granite  at,  correlation  of 65 

dikes  in. 115 

distribution  of 266 

exposures  of '. 266 

metamorphic  effects  of 273 

Ogishke  conglomerate  contact  with 374 

petrographic  characters  of 266-267 

relations  of. 268-273 

topography  of 266 

Ogishke  conglomerate  at 85-86,374,309,311 

relation  of  topography  and  geology  at 434 

syenite  at 101 

Upper  Kewatin  rocks  at 119-120 

Saganaga  gneiss,  stratigraphic  place  of... 89 

Saganaga, granite,  relation  of,  to  Kewatin  rocks  .,^      109 
to  Ogishke  conglomerate 305 


Page. 

Saganaga  syenite,  age  of . 101 

Saganaga  syenite  conglomerate,  origin  of. 94 

St.  Cloud  granite,  age  of _ 76 

St.  Louis  River,  geology  of  region  near 73 

St.  Louis  River,  rocks  near _ 70 

St.  Paul  and  Duluth  Railroad,  gabbro  along... 402 

Sardeson,  F.  W.,  report  of.  cited 116-117 

Savoy  iron  mine,  reference  to 217 

Sawtooth  Hills,  location  of 38 

Sawtooth  Mountains,  origin  of  name  of 19 

Schistose   greenstone,  occurrence   and   character 

of. 153-154 

Schists,  occurrence  of 253-254 

Scrammers  in   mine   iising  caving   system,  view 

showing 240 

Seagull  Lake,  syenite  at 86 

Seagull  syenite  conglomerate,  origin  of 94 

Sericite  schists,  occurrence  of 252 

Sibley  iron  mine,  reference  to 217 

Silver  City,  location  of. 53 

Silver-bearing  rocks,  Vermilion  Lake,  occurrenceof .        67 

Slate,  folded,  plate  showing 178 

Slaty  cleavage,  plate  showing. 178 

Smyth,  H.  L.,  and  Finlay,  J.  R.,  citation  from,  on 

iron  ores,  origin  of 232 

on  Ogishke  conglomerates,  origin  of 286 

on  Jasper  at  Soudan  Hill 230 

on  pseudo-conglomerates  _ 253 

on  schist  at  Lee  mine. 223 

on  slates  of  Vermilion  district 381 

on  Vermilion  range,  geology  of 111-112 

figure  cited  from '_ 231 

Smyth,  H.  L.,  Bayley,  W.  S.,  and  Van  Hise,  C.  R., 

cited  on  iron  ores,  origin  of 232 

Snowbank  granite,  age  of 406 

distribution  of 361 

exposures  of 315,361,364 

intrusion  of 441 

metamorphism  of  slates  by 341 

occurrence  of 24, 120 

petrographic  characters  of 361-362 

relations  of,  to  Duluth  gabbro 363-364, 407 

to  Keweenawan  gabbro 363-364 

to  Lower  Huronian  sediments 363 

topography  of 361 

Snowbank  Lake,  conglomerate  at  and  near  ..  305,315,318 

geology  of  region  near 110-111 

granite  at  and  near 87,110-111,315,364 

Keweenawan  gabbro  at 315 

Knife  Lake  slates  at 341 

Lower  Huronian  sediments  at 398, 300 

metamorphism  at  and  near 315, 350 

Soil  of  the  district,  character  of 50 

Sondan,  graphitic  slates  at 180 

iron-ore  deposits  at 23, 215, 234-335, 243, 244 

location  and  population  of 52 

mining  methods  at ^..  334-339 

sections  in  mines  at 235,236,237,238 

Soudan  formation,  age  of 33,195-196,200 

character  of 179-188 

dikes  in 356 

distribution  of- ..' 173-173 

divisions  of 179 

exposures  of 173-175 

features  of 31-33 

folding  in 175-177,310,312 

figure  showing 205 


460 


INDEX. 


Page, 

Soudan  foiination.  gi-anite  dikes  in 356 

iron  ores  of - - 183-185 

life  content  of -  - 438 

microscopic  characters  of  fragmental  part  of.  180-181 

origin  of -_  188-191,438-439 

petrogi-aphic  characters  of 179-188 

relations  of .  to  adjacent  formations --. 191-200 

to  Arcliean  intrusives 199 

to  basic  eruptives -. ---      200 

to  Ely  greenstone.-- 205-207 

to  Giants  Range  gi-anite ---  356,359 

to  intrusive  rocks --- 256 

to  Lower  Huronian,  Vermilion  Lake  area.  281-283 

to  Ogishke  conglomerate.- - 206-209 

stratigraphic  position  of -  33,195-196 

sti-ucture  of -  175-179 

thickness  of 200 

topogi'aphy  of- 175 

Soudan  Hill,  conglomerate  at --- 287 

graphitic  slate  at -  - 179 

intrusive  rocks  in  iron-beai*ing  formation  at 199 

iron-bearing   formation   and   associated  rocks 

at - 201-202 

Knife  Lake  slates  at 295-296 

jasper,  banded,  at 230 

jasper  replaced  by  iron  ore  at - 231 

Soudan  formation  at 173,175 

structure  of .-. -_ 224-225 

topography  and  geology  at,  relation  of 431 

Specular  iron  ore,  occuia-ence  of 67 

Spherulites,  occurrence  of 167-168 

Spherulitic  structure  in  Ely  greenstone . .  -  141-144, 146-148 
Spherulitic  texture  in  greenstones,  plate  showing.-      142 

Spilosites,  occurrence  of - 345 

Spurr,  J.  E.,  cited  on  Biwabik  formation 378 

citation  of,  on  iron-bearing  rocks 115,385 

on  iron-bearing  rocks,  origin  of 190 

Stratigi-aphy  of  the  district,  table  showing 33 

Streams  of  the  district 40-41 

Streng,  A.,  and  Kloos,  J.  H.,  cited  on  geology  of  the 

district .  - 70 

Stuntz,  Ct.  R.,  exploratory  work  by 214 

Stuntz  Bay,  Vermilion  Lake,  conglomerates  at 252, 

291-292 

naming  of  - - 214 

Stuntz  conglomerate,  relations  of,  to  Ogishke  con- 

glomerat  e 88 

Stuntz  Island,  conglomerate  at 287 

dikes  on - -- -,.  257,373 

granite-porphyry  at 257 

Sucker  Lake,  Agawa  formation  at  - 325 

Sucker  Point,  Indian  reservation  at -..  20,54 

Knife  Lake  slates  at -- -- 295 

Swede  Bay,  Knife  Lake  slates  at ; 295 

Taconic  system,  components  of 76,120 

That  Mans  Lake,  Agawa  formation  at 325,326,329,330 

The  Other  Mans  Lake.  Agawa  formation  at 325, 

:«6.:$».:^30 

This  Mans  Lake,  Agawa  formation  at 325, 

326,:J2tt.:^^,;«l,332 
Thompson,  David,  cited  on  travels  in  the  North- 
west         63 

Thunder  Bay,  Animikie  series  at 74 

Titaniferous  magnetite,  occurrence  of 2U 


Page. 
Todd,  J.  E.,  cited  on  glacial  deposits  in  Minne- 
sota  _._ 425.426 

Topography  of  the  district,  features  of  . .- 34^46 

relations  of,  to  geologic  structure -  4;:Jl-i:36 

Tower,  conglomerate  at 387,292 

greenstones  and  jasper  folded  near 211 

gi-aphitic  slatesat  - 179 

gi'eenstones  near  ,_ - 211 

iron-bearing  formation   and  associated   rocks 

at.. 2((U-201 

iron-ore  deposits  at 23 

location  of 52 

Lower  Huronian  rocks  at 277 

metamoi-phosed  slates  near 349 

ore  depositsat 243,244 

population  of _ 20.52 

railroad  built  to 22,29 

slatesat 292 

Soudan  formation  at. 173,200-201 

Tower  group,  stratigraphic  place  of -       8S 

Tower  Hill,  jasper  near 182 

greenstones  and  jaspers  at,  relations  of 202-203 

iron-bearing  formation  and  associated  rocks  at-      201 

Soudan  formation  at 1 73, 175 

topogrj,phy  and  geology  at,  relation  of 431 

Townline  Lake,  jasper  reported  to  occur  at 310 

Towns  in  the  disti-ict.  location  and  population  of.  20,52-54 

Township  55  North,  Range  11  "West 123 

T.58N.,R.10W - m 

T.60N..R.6W ...- - — .      123 

T.60N.,R.7  W.-- 123 

T.60N..R.14  W 429 

T.60N..R.15W- - 429 

T.60N.,R.16'W -, 429 

T.61  N.,R.  13  W., section  6 --      427 

T.61  jSr.,R.14W ----  428,429 

sections  1,2 ■ 427 

section  4  . .- --_      175 

section  lU 156,427 

sections  11, 12, 14. 15,16, 18. 19.21,22,29, 

30  -- -.-      427 

T.61N.,R.15W - 131,428,429 

section  1-- 197,203 

section  2 197 

sections -.,_  174,197.203 

section  4 174 

section6 287.292 

sections  21-28 427 

T.61N.,R.Ui  W 277,429 

T.62N.,R.2W..- - -.--      123 

T.62N..R.4W- 12:3 

T. 62  N.,R.  10  "W..  sections -... 39S 

section  :30 - 119 

T.62N.,R.ll  W- - -- 354 

T.62N.,R.12  W 35.354 

section  1 -...  3(iO,4;i6 

section  2 - -  37.;360 

section  3 16»j.  11)9.  mi  a59.  m).  4;J6 

section  4 199,20i),359 

section  7 a59.:360 

stctionS 360 

section  9 436 

section  11 354.4^36 

section  17 .' 360.436 

sectitm  IS.. 199.:*M) 

section  19 167.359..M) 

section  :30 37 


INDEX. 


461 


Paffe. 

T.fi2N.,E.13W --      428 

section  3 -. - 166 

section  4 132,297,332 

section? -.-  195,212 

section  14 427 

section  15 194,427 

section  17 166,188 

section  18 ., — 171 

sections  22,23 427 

section  24 167,a59,360 

sections  26,27 436 

section28 ..-. 171,359,360,436 

section  29 171,436 

sectional.. 171, .353 

section  32 171,194,353 

T.  62  N.,  R.  14  W 166,428.429 

section  2 277 

sections 435 

section5 204 

section  0 174,204,2.55 

section? 174,175,199,204,257 

section  8 174 

section9 2.56 

section  10 212,256 

section  15 131,277 

sections  16-18.. 131 

section  20' 256 

section  21. 1.31,197,256 

section27 2.56 

section  35 168 

T.li2N.,  E.  15W 173,428 

section  1 1.31 ,  174, 199, 204, 255, 256 

section  2 1,31,433 

sections  .3,  4 131 

section? 131,2.56 

sections  8, 9, 12 131 

section  14 287 

section  20 202,295 

section  22 373 

section  27... 200 

section  28 201 

section  .35 36,131 

section  36 131 

T.  62  N.,  R.  WW 277 

T.63N.,  R.  IW 123 

T.  63N.,  B.  9W 133,361,427,432 

section  4. 172,192,205 

sections 263 

section  6 142,168 

section  10.... 341 

section  15 4.35 

section  16 316 

section  17 133,  .316, 341, 348, 373 

section  19 353,357,:i58 

section  2u 169,398,316 

T.  63N.,  R.  low 133 

section  1 142 

sections  3-.5,  8,9 427-428 

section  10 142, 167, 189, 199, 360, 427-428 

section  11 142,167 

section  12 142,168 

sections  14,  15 142,.360 

sections  16, 17 360 

section  20 ._ 166,360 

section  21 _. 360 

section24 353,355 


T.63N. 


T.63N. 


T.  63  N, 


T.63N. 

T.63N. 

T.  63N. 
T.  64  N. 
T.  64  N. 

T.  64  N. 
T.  84N. 
T.  64N, 
T^  64  N, 


T.  64  N, 


T.  64N 
T.  65  N 


Page. 

R.  low.,  section  25 353 

section  26 398,435 

section  29 298,359 

section:*).. _. 340,:360 

section31 360 

section  34 357 

section  :i5 398 

,  R.U  W 173 

section  6 148 

section  7 148,192 

section  26 340 

section  30 : 168;  173, 174, 175, 427 

section  31 427,436 

section  Si _ 357 

section  36 i .360 

,  R.  12  W.,  section  9 146,147 

section  10- 142,147 

section  11 142 

section  13 192 

sections  14-16 142 

section  21 142,263 

section  22 142 

section  25 173,174,175,212,427 

section  26... 427 

section  30 164 

sections  34-86 427 

,  R.  13  W.,  sections  20,  30. 263 

section  .32. 435 

,  R.  15  W 435 

sections  34,  35 433 

.  R.  16  W 277 

,  R.  2  W.... 123 

.  R.5  W 3-52 

section  6 351,422,433 

,  E.  6W 432 

section  1 161,351,4.33 

section  2 161 

,  R.  7W ^ 433 

section  1 365,369 

section  2 365 

,  R.8  W 361 

section  11 349 

sections  26,  35 161 

,  R.  9W 133,361,428,432,433 

section  10 : 132 

section  16.. 132,173,261,263,337 

section  17 2a3 

section  19 , 407 

section  32 146 

section  33 146,207 

section  36 362 

,  R.  low 428 

section  19 168,263 

section  31 132 

,,  R.  12  W.,  section  36.. 4.35 

,,JR.  4  W.,  section  21 375,377 

section  22 ._. 375,377 

section  23 375,387,390 

section24 387 

section  28 375,390,391 

section27 119,  ia3, 161, 37.5, 377. 388 

section  28 34,  .35, 181, 398 

section29 : 161 

section  30 181,298,388 

section  34 377 


4(32 


INDEX. 


Page. 

T.  fwN.,  R.  5  W - 352 

section  ij 269 

section  7 .- _ 271 

sections  22,  23 274 

sections.) .-- 388 

sections  26,  27 389 

sections  29,  30 .,      398 

sections  31,  32 433 

section  Si —  -      390 

section  34.. 119,377,435 

section  35 435 

T.&-)N.,  R.  OTV 432 

sections  3,10 434 

sections  11,18.19,25.... 133 

section  26.. 133,321,323 

section  27 133.302,321,323 

section  28 319 

section  29 325,332 

section30 325,332 

section  31.. 133 

section  35 301 

T.  6oN.,  R.  7W 432 

section  21 133 

sections  22. 23 433 

sections  25, 36 133 

T.  66  N.,  R.  5  W 428 

section  ;30 : 269,434 

T.  66  N..  R.  6  W - 428 

sections  24. 25, 26, 34 _ 434 

section  35 301,434 

T.  66  N..  R.  12  W..  section" 37 

T.  66  N.,  R.  13  "W.,  sections  11, 12 37 

Travel  in  the  distrir-t.  routes  and  metliods  of ..  20-21,56-63 
Trout,  Burntside,  and  Basswood  lakes,  granites  of, 

age  of 261 

distribution  of 258 

exposures  of 258,262-263 

folding  in 262 

petrographic  characters  of 259-260 

relations  of 260-261 

topography  of 259 

Tuffs,  occurrence  of.  associated  with  greenstones..      166 
Twin   Lakes,    ellipsoidal    spherulitic    greenstones 

near - 168 

Twin  Peaks,  greenstones  at 133 

greenstone  and  gabhro  in  contact  at 162 

Twin  Peaks  Ridge,  conglomerate  and  greenstone  in 

contact  at 303-304 

TJ. 

Upham.  Warren,  cited  on  glacial  deposits  in  Min- 
nesota       426 

Upper  Huronian  sediments,  area  of 25, 374 

character  of -. 25-26 

deposition  of.  conditions  of  ,. 442-443 

formations  composing _..,  25,33 

Gunflint  formation  of 374-390 

metamoi*phism  of 25.26,419 

sediments,  relation  of.  to  Duluth  gabbi-o. 407 

to  Logan  sills 408^410 

to  Lower  Huronian 97-98 

Rove  slate  of 390-395 

See   also  Animikie,    Biwabic,    Gunfiint.    and 
Rove. 
Upper  Kewatin  rocks,  occurrence  and  chai'acter 

of 119-120 


Page. 

Van  Hise,  C.  R..  acknowledgments  to 56 

citation  from,  on  Animikie  rocks,  age  of 125 

on  Archean  and  Algonkian  rocks 1(^HJ6. 129 

on  zone  of  fracture  in  rocks 148 

on  fissility  and  bedding,  relations  of 280 

on  geology  of  Lake  Superior  region  ..  97-9S, 

105-106, 127-12S 

on  grunerite.  origin  of 1S7 

on   hornblende   fragments,    secondary   en- 
largement of 77,319.338 

on  iron-ore  deposits  of  Lake  Superior  region, 

origin  of 127-128,190-191.228-230 

on  metamorphism 161 

on  schistosity  and  cleavage  as  related  to 

bedding ;3(J2 

on  slaty  cleavage 177 

geologic  work  in  Vermilion  district  done  by 30 

Van  Hise,  C.  R.,  and  Irving.  R.  D.,  citation  from,  on 

Animikie  slates 396 

on  Gunflint  beds.. - 379 

on  iron  ores,  origin  of 228 

on  mica-schist  at  English  Lake 350 

on  Penokee  series 97-390 

Van  Hise,  C.  R..  Bayley,  W.  S.,  and  Smyth,  H.  L.. 

citation  from,  on  iron  ores,  origin  of 232 

Van  Horn,  F.  B.,  acknowledgments  to 31 

Vermilion  gi-oup,  relations  of  rocks  of 87 

Vermilion  Lake,  area  of 41 

character  of 114 

conglomerate  at 291-292 

stratigraphlc  relations  of ." 98-114 

conglomerates  and  pseudo-conglomerates  at .  251-252 

depth  of 45 

geology  of  region  near 65,68.69 

glacial  deposits  at 428 

gold-bearing  rocks  near 67-69 

gold  mining  at 213 

gi*anite  at,  age  of 68 

distribution  of 247 

exposures  of 247-248 

folding  in 250-251 

petrographic  characters  of 248-250 

relations  of,  to  Ely  greenstone 255-256 

structure  and  metamorphism  of 251-254 

greenstones  at  and  near 118,131.135 

str uctiire  of.. _ 135 

hematite  ores  near 67,83 

iron-bearing  rocks  at,  relations  of 74,2(.e 

iron  ores  at  and  near 67,75,83,213.214 

jasper  and  hematite  near 72 

Knife  Lake  slates  at 295 

Lower  Huronian  rocks  at  and  near,  age  of.. —      284 

distribution  of 277-278 

divisions  of 275-276 

exposures  of 278 

Knife  Lake  slates  of 293-296 

Ogishke  conglomerate  of 284-293 

relations  of 281-284 

structure  of 278-281 

topography  of - 278 

micaceous  rocks  near 68 

ores  near ^ 

origin  and  general  features  of 44 

pseu  do -conglomerates  at 251-25:J 

schists  at --  253-254 

sedimentary  rocks  at 23 


INDEX. 


463 


PagL'. 

Vermilion  Lake,  silver -bearing  rocksnear _        67 

slate  and  jasper  banding  near _.      178 

Soudan  formation  at _      173 

Stimtz  conglomerate  at 114 

succession  of  rocks  at 79-i5() 

topography  and  geology  at,  relations  of. . .  432-433, 435 
Upper  Keewatin  rocks  at  ._ 119-120 

Vermilion  group,  relations  of  -  _ 87,91-92 

str atigr apbic  place  and  thickness  of 89 

Vermilion  iron  range,  explanation  of  use  of  term. ,        34 

Vermilion  moraine,  extent  and  general  features 

of. 426-429 

map  showing 427 

Vermilion  range,  geology  of 111-112 

Vermilion  River.    See  Pike  River. 

Volcanic  activity,  periods  of 437-447 


A\" 


Walcott,  C.  D.,  cited  on  Cambrian  Sea  transgression 

in  North  America 445 

Watab  granite,  age  of 76 

Water  power  in  the  district 42-43 

Weidman,  Samuel,  cited  on  ellipsoidal  structure  in 

Wisconsin  rocks '. 150 

West  Gull  Lake,  glacial  deposits  near 428 

granite  and  greenstone  at 274 

greenstone-conglomerate  contact  at 324 

map  shovring  exposures  at 272 

rocks  at „.. 271-273 

West  Seagull  Lake,  conglomerate  and  syenite  at. ..       86 

granite  at 115 

West  Two  Rivers,  conglomerate  at 292-293 

White  Iron  gneiss,  stratigraphic  place  o£ 89 

White  Iron  Lake,  granite  at 353-354 

granitoid  and  gneissoid  rocks  at 87 

greenstones  near 118 

Whitewood  Lake.    See  Basswood  Lake. 
Whittlesey,  Charles,  cited  on  geology  of  the  dis- 
trict   68,70 

Willis,  Bailey,  cited  on  succession  and  structui-e  in 

part  of  Vermilion  district 79-80 

work  in  district  done  by 29-30, 214 

Willmott.  A.  B.,  cited  on  ellipsoidal  structure  in 

Lake  Superior  rocks 150 

work  in  district  done  by 125 

Winchell,  Alexander,  citation  from,  on  Archean 

rocks  of  Minnesota 95-96 

on  conglomerates 85-86,94 

on  geology  of  northeastern  Minnesota.  81-82, 85-89 

on  granite  of  Saganaga  Lake 258, 265 

on  jasper  at  Townline  Lake 310 

on  muscovado 343 

on  schist  and  granite  at  Burntside  Lake,  re- 
lations of 158 

on  succession  of  terranes  in  northeastern 

Minnesota 88-89 

on  uncomformities  in  Vermilion  district 85-89 


Page. 
Winchell,  H.  V.,  citation  from,  on  granite  of  Sagan- 
aga Lake. 265,268 

on  inclusions  of  iron-bearing  formation  in 

greenstones 197 

on  iron-bearing  rocks  *of    Gunflint  forma- 
tion  ._ 385 

on  iron  oxide  bodies  in  Duluth  gabbro 420 

on  iron  region  of  Minnesota 93 

on  muscovado. _  _ ,      343 

on  Saganaga  syenite 101 

Winchell,     N ,     H . ,     citation    from,    on    Archean 

rocks 70 

on  arkoses  of  Saganaga  Lake .      270 

on  basalt,  ellipsoidal,  in  Minnesota 145 

on  conglomerate  of  Ogishke  Muncie  Lake.,      72, 

304,321 

on  copper -bearing  series 72 

on  crystalline  rocks  of  Minnesota 106-107 

on  Duluth  gabbro 401,408 

on  Ely  greenstones,  origin  of 154-155 

on  eruptive  rocks  of  Minnesota 94 

on  geology  of  Minnesota 70,71-73,76-77, 

82-a5, 90-92, 94.  lOO-lOl.  104, 106-107, 113-114, 117-125 

on  Giants  and  Mesabi  ranges 35 

on  glacial  lakes 127,430 

on  gold-bearing  quartz  at  Long  Lake 164 

on  great  gabbro 100-101 

on  iron  ore  at  Vermilion  Lake 69,76 

on  iron  ores  of  Gunflint  beds 385 

on  Kawishiwin  agglomerate 104 

on  Mesabi  and  Giants  ranges 35 

on  metamorphism  of  slates  to  schist 296 

on  Ogishke  conglomerate   and  Ely  green- 
stone, contact  of 304 

on    schists    and     granites    at     Burntside 

Lake 158 

on  Snowbank  granite,  origin  of 363 

on  spherulitic  mottling  of  Ely  greenstones  .      142 

on  structural  geology  of  Minnesota 117-125 

maps,    geologic,    collected    and    published 

by... 12T 

Winchell,  N.  H.  and  H.  V.,  citation  from,  on  rocks 

of  the  Biwabik  formation 378 

on  iron  ores  of  Minnesota 94-95, 

96-97, 101-103. 189-190. 233 
Winchell.  N.  H.,  Grant,  U.  S.,  and  Elftman,  A.  H., 
report  prepared  by,  on  geology  of  the 

Vermilion  district 117 

Wind  Lake,  Agawa  formation  near.  325, 326, 829, 330, 333-335 

conglomeratic  rock  at 170-171 

green  schist  near 170 

Winten,  industries  of 20 

location  and  population  of 53 

Wonder  Island,  conglomerate  at 85-86 

Zenith  iron  mine,  reference  to... 317 


O 


PUBLICATIONS  OF  UNITED  STATES  GEOLOGICAL  SURVEY. 

[Monograph  XLV.] 

The  serial  publications  of  the  United  States  Geological  Surrey  consist  of  (1) 
Annual  Reports,  (2)  Monographs,  (3)  Professional  Papers,  (-4)  Bulletins,  (5)  Mineral 
Resources,  (6)  Water-Supply  and  Irrigation  Papers,  (7)  Topographic  Atlas  of  the 
United  States — folios  and  separate  sheets  thereof,  (8)  Geologic  Atlas  of  the  United 
States — folios  thereof.  The  classes  numbered  2,  7,  and  8  are  sold  at  cost  of  publica- 
tion; the  others  are  distributed  free.  A  circular  giving  complete  lists  ma}'  be  had 
on  application. 

MONOGRAPHS. 

I.  Lake  Bonneville,  by  G.  K.  Gilbert.     1890.     4°.     xx,  4.38  pp.     51  pi.     1  map.     Price  $1.50. 

II.  Tertiary  history  of  the  Grand  Canon  district,  with  atlas,  by  C.  E.  Button,  Capt.,  U.  S.  A.     1882.     4°. 
xiv,  264  pp.     42  pi.  and  atlas  of  24  sheets  folio.     Price  $10. 

III.  Geology  of  the  Comstock  lode  and  the  Washoe  district,  with  atlas,  by  G.  F.  Becker.     1882.     4°. 
XV,  422  pp.     7  pi.  and  atlas  of  21  sheets  folio.     Price  $11. 

IV.  Comstock  mining  and  miners,  by  Eliot  Lord.     1883.     4°.     xiv,  451  pp.     3  pi.     Price  S1.50. 

V.  The  copper-bearing  rocks  of  I  fke  Superior,  by  E.  D.  Irving.    1883.    4°.    xvi,  464  pp.    15  1.    29  pi. 

and  maps.     Price  $1.85. 

VI.  Contributions  to  the  knowledge  of  the  older  Mesozoic  flora  of  Virginia,  by  AV.  M.  Fontaine.     1883. 
4°.     xi,  144  pp.     54  1.     54  pi.     Price  $1.05. 

VII.  Silver-lead  deposits  of  Eureka,  Nevada,  by  J.  S.  Curtis.     1884.     4°.     xiii,  200  pp.     16  pi.     Price 
$1.20. 

VIII.  Paleontology  of  the  Eureka  district,  by  C.  D.  Walcott.     1884.     4°.     xiii,  298  pp.     24  1.     24  pi. 
Price  $1.10. 

IX.  Brachiopoda  and  Lamellibranchiata  of  the  Earitan  clays  and  greensand  marls  of  New  Jersey,  bv 
E.  P.  Whitfield.     1885.     4°.     xx,  338  pp.     35  pi.     1  map.     Price  $1.15. 

X.  Dinocerata.     A  monograph  of  an  extinct  order  of  gigantic  mammals,  by  0.  C.  Marsh.     1886.     4°. 
xviii,  243  pp.     56  1.     56  pi.     Price  $2.70. 

XI.  Geological  historv  of  Lake  Lahontan,  a  Quaternary  lake  of  northwestei'n  Nevada,   bv  I.   C. 
Eussell.     1885.     4°.     xiv,  288  pp.     43  pL  and  maps.     Price  SI. 75. 

XII.  Geology  and  mining  industry  of  Leadville,  Colorado,  with  atlas,  by  S.  F.  Emmons.     1886.     4°. 
xxix,  770  pp.     45  pi.  and  atlas  of  35  sheets  folio.     Price  $8.40. 

XIII.  Geology  of  the  quicksilver  deposits  of  the  Pacific  slojje,  with  atlas,  by  G.  F.  Becker.     1888. 
4°.     xix,  486  pp.     7  pi.  and  atlas  of  14  sheets  folio.     Price  $2. 

XIV.  Fossil  fishes  and  fossil  plants  of  the  Triassic  rocks  of  New  Jersev  and  the  Connecticut  Valley, 
by  J.  S.  Newberry.     1888.     4°.     xiv,  152  pp.     26  pi.     Price  $1. 

XV.  The  Potomac  or  younger  Mesozoic  flora,  by  W.  M.  Fontaine.     1889.     4°.     xiv,  377  pp.     180  pi. 
Text  and  plates  bound  separately.     Price  S2.50. 

XVI.  The  Paleozoic  fishes  of  North  America,  by  J.  S.  Newberrv.     1889.     4°.     340  pp.     53  pi.     Price 
$1.00. 

XVII.  The  flora  of  the  Dakota  group,  a  posthumous  work,  bv  Leo  Lesquereux.     Edited  bv  F.  H. 
Knowlton.     1891.     4°.     400  pp.     66  pi.     Price  $1.10. 

XVIII.  Gasteropoda  and  Cephalopoda  of  the  Earitan  clays  and  greensand  marls  of  New  Jersej',  bv 
R.P.Whitfield.     1891.     4°.     402  pp.     50  pi.     Price  $1. 

XIX.  The  Penokee  iron-bearing  series  of  northern  Wisconsin  and  i\Iichigan,  bv  E.  D.  Irving  and 
C.  E.  Van  Hise.     1892.     4°.     xix,  534  pp.     Price  $1.70. 

XX.  Geology  of  the  Eureka  district,  Nevada,  with  an  atlas,  by  Arnold  Hague.     1892.     4°.     xvii,  419 
pp.     8  pi.     Price  $5.25. 

XXI.  The  Tertiary  rhynchophorous  Coleoptera  of  the  LTnited  States,  by  S.  H.  Scudder.     1893.     4°. 
xi,  206  pp.     12  pi.     Price  90  cents. 

XXII.  A  manual  of  topographic  methods,  bv  Henry  Gannett,  chief  topographer.     1893.     4°.     xiv, 
300  pp.     18  pi.     Price  $1. 

XXIII.  Geologv  of  the  Green  Mountains  in  Massachusetts,  bv  Raphael  Pumpelly,  T.  N.  Dale,  and 
J.  E.  Wol£     1894.     4°.     xiv,  206  pp.     23  pi.     Price  $1.30.' 

MON    XLV — 03 30  I 


II  PUBLICATIONS    OF    UNITED    STATES    GEOLOGICAL    SURVEY. 

XXIV.  Mollusca  and  Crustacea  of  the  Miocene  formations  «i  Xew  Jersey,  bv  R.  P.  Whitfield.  1894. 
4°.     19.S  pp.     24  pi.     Price  90  cents. 

XXV.  The  Glacial  Lake  Agassiz,  by  Warren  Uphani.     1S9.5.     4°.     xxiv,  658  pp.     38  pi.     Price  SI. 70. 

XXVI.  Flora  of  the  Ambov  clavs,  by  J.  S.  Xewberrv;  a  posthumous  work,  edited  bv  Arthur  Hollick. 
1S9.^.     4°.     260  pp.     58  pi.  ■  Price  81. 

XXVII.  Geology  of  the  Denver  Basin  in  Colorado,  bv  S.  F.  Emmons,  Whitman  Cross,  and  G.  H. 
Eldridge.     1896.     4°.     .556  pp.     31  pi.     Price  SI. .50. 

XXVIII.  The  Marquette  iron-bearing  district  of  Michigan,  with  atlas,  by  C.  R.  Van  Hise  and  W.  S. 
Baylev,  including  a  chapter  on  the  Repul;)lic  trough,  by  H.  L.  Smyth.  1895.  4°.  608  pp.  .35 
pi.  and  atlas  of  39  sheets  folio.     Price  So.  75. 

XXIX.  Geology  of  old  Hampshire  County.  Mas.?achusetts,  comprising  Franklin,  Hampshire,  and 
Hampden  counties,  by  B.  K.  Emerson.     1898.     4°.     xxi,  790  pp.     35  pi.     Price  81.90. 

XXX.  Fossil  Medusce,  by"  C.  D.  Walcott.     1898.     4°.     ix,  201  pp.     47  pi.     Price  81.50. 

XXXI.  Geology  of  the  Aspen  minins  district,  Colorado,  with  atlas,  by  J.  E.  Spurr.  1898.  4°.  xxsv, 
260  pp.     43' pi.  and  atlas  of  30  sheets  folio.     Price  S3.60, 

XXXII.  Geology  of  the  Yellowstone  National  Park,  Part  II,  descriptive  geology,  petrography,  and 
paleontology,  bv  Arnold  Hague,  J.  P.  Iddings,  W.  H.  Weed,  C.  D.  Walcott,"  G.  H.  Girtv,"T.  W. 
Stanton,  and  F.  H.  Knowlton.     1899.     4°.     xvii,  893  pp.     121  pi.     Price  82.45. 

XXXIII.  Geology  of  the  Xarragansett  Basin,  by  N.  S.  Shaler,  J.  B.  Woodworth,  and  A.  F.  Foerete. 
1899.     4°.     x"x,  402  pp.     31  pi.     Price  81. 

XXXIV.  The  glacial  gravels  of  Maine  and  their  associated  deposits,  by  G.  H.  Stone.  1899.  4°.  xiii, 
499  pp.     .52  pi.     Price  81. .30. 

XXXV.  The  later  extinct  floras  of  Xorth  America,  by  J.  S.  Newberry;  edited  by  Arthur  Hollick. 
1898.     4°.     xviii,  295  pp.     68  pi.     Price  81.25. 

XXXVI.  The  Crystal  Falls  iron-bearing  district  of  ^Michigan,  by  J.  !M.  Clements  and  H.  L.  Smyth; 
with  a  chapter  on  the  Sturgeon  River  tongue,  bv  AV.  S.  Bavlev,  and  an  introduction  bv  C.  R.  Van 
Hise.     1899.     4°.     xxxvi,  512  pp.     .53  pi.     Price  .82. 

XXXVII.  Fossil  flora  of  the  Lower  Coal  Measures  of  Missouri,  by  David  White.  1899.  4°.  xi,  467 
pp.     73  pi.     Price  81.25.  ■ 

XXXVIII.  The  Illinois  glacial  lobe,  by  Frank  Leverett.     1899.     4°.     xxi,  817  pp.    24  pi.    Price  81.60. 
.XXXIX.  The  Eocene  and  Lower  Oligocene  coral  faunas  of  the  United  States,  with  descriptions  of  a 

few  doubtfully  Cretaceous  species,  by  T.  W.  Vaughan.     1900.     4°.     263  pp.     24  pi.     Price  81.10. 
XL.  Adephagous  and  clavicorn  Coleoptera  from  the  Tertiary  deposits  at  Florissant,  Colorado,  witli 

descriptions  of  a  few  other  forms  and  a  systematic  list  of  the  non-rhvncophorous  Tertiary  Coleoptera 

of  Xorth  America,  by  S.  H.  Scudder.    "1900.     4°.     148  pp.     11  pis.     Price  80  cents. 
XLI.  Glacial  formations  and  drainage  features  of  the  Erie  and  Ohio  basins,  by  Frank  Leverett.     1902. 

4°.     802  pp.     26  pis.     Price  81.75. 
XLII.  Carboniferous  ammonoids  of  America,  by  J.  P.  Smith.     1903.     4°.     211  pp.     29  pis.     Price 

85  cents. 
XLIII.  The  Mesabi  iron-bearing  district  of  Minnesota,  bv  C.  K.  Leith.     1903.     4°.     316  pp.     33  pis. 

Price  81.50. 
XLIV.  Pseudoceratites  of  the  Cretaceous,  by  Alpheus  Hyatt,  edited  by  T.  \V.  Stanton.     1903,     4°. 

350  pp.     47  pis.     Price . 

XLV.  The  Vermilion  iron-bearing  district  of  Minnesota,  with  atlas,  bv  J.  M.  Clements.     1903.     4°. 

463  pp.     13  pis.     Price . 

All  remittances  must  be  by  MOXEr  order,  made  payable  to  the  Director  of  the 
United  States  Geological  Survey,  or  in  currency — the  exact  amount.  Checks,  drafts, 
and  postage  stamps  can  not  be  accepted.     Correspondence  should  be  addressed  to 

The  Director, 

United  States  Geological  Survey, 

Washington.  D.  C. 


LIBRARY  CATALOGUE  SLIPS. 

[Mount  each  slip  upon  a  separate  card,  placing  the  subject  at  the  top 
of  the  second  slip.  The  name  of  the  series  should  not  be  repeated 
on  the  series  card,  but  add  the  additional  numliers,  as  received,  to 
the  first  entry.] 


Clements,  J[ulius]  Morgan. 

.  .  .  The  Vermilion  iron-bearing  district  of  Min- 
nesota, with  an  atlas;  by  J.  Morgan  Clements. 
Charles  Richard  Van  Hise,  geologist  in  charge. 
Washington,  Gov't  print,  off.,  1903. 

(U.  S.  Geological  survey.      Monographs  v.  xlv. )     463  p.     13  pi. 
30""'.  and  atlas  (26  sheets)  53x46"=°'. 


Clements,  J[ulius]  Morgan. 

.  .  .  The  Vermilion  iron-bearing  district  of  Min- 
nesota, with  an  atlas;  by  J.  Morgan  Clements. 
Charles  Richard  Van  Hise,  geologist  in  charge. 
Washington,  Gov't  print,  off.,  1903. 

(U.  S.  Geological  survey.     Monographs  v.  xlv.)      463  p.      13  pi. 
30""'.  and  atlas  (26  sheets)  .53x46™'. 


U.  S.  Geological  survey. 

Monographs. 
V.  45.  Clements,  J.  M.     The  Vermilion  iron-bearing 
district  of  Minnesota,  with  an  atlas.     1903. 


U.  S.  Dept.  of  the  Interior. 
see   also 
U.  S.  Geological  survey. 


